docs: say to use one SignedXml instance per signature

[cjbarth]

Fixes #409.

#409 reused one SignedXml and called addReference() for every request, so each new signature carried one more Reference. The README doesn't say whether an instance can be reused. This adds a short "One instance per signature" section under the SignedXml API: create a new instance for each signature you create or verify, including each signature in a document that has more than one. It also describes the two kinds of state an instance keeps.

I measured both on master:

usage	result
one instance, addReference() once, three documents signed	one Reference each, all verify
one instance, addReference() before each of three documents	1, 2 and 3 Reference elements
one verifier, three documents checked	each check passes; getSignedReferences() returns 1, 2, then 3 entries, including the earlier documents' content
one instance, two addReference() calls, then checkSignature()	getReferences() holds only the checked signature's one reference
one verifier: pass, pass, fail, pass	getSignedReferences() returns A; then A, B; then empty; then D

The verifier rows matter most. getSignedReferences() is how the README tells callers to get the content a signature covers. On a reused verifier, it also returns content from documents checked earlier.

#409 also suggested making a repeated reference throw. That would reject calls that work today, so this PR changes only the documentation.

Docs only: npm run lint passes.

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Summary by CodeRabbit

• Documentation
  • Recommended creating a separate SignedXml instance for each signature, including when a document contains multiple signatures.
  • Clarified that computeSignature() signs all references held by the instance, and that adding references to a reused instance affects later signatures.
  • Documented how checkSignature() updates held references, and how successful checks accumulate references returned by getSignedReferences() until a failed check clears them. On a reused instance, the results may include content covered by earlier signatures.

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📝 Walkthrough

Walkthrough

The README recommends a separate SignedXml instance for each signature. It describes how signing and verification calls affect the instance’s references and signed-reference results.

Changes

SignedXml instance reuse

Layer / File(s)	Summary
Document instance reuse guidance README.md	The README recommends a separate instance for each signature. It explains that signing uses accumulated references, verification replaces them with references from the checked signature, and signed-reference results persist after successful checks until a failed check clears them.

Priority: ⬇️ Low

Estimated code review effort: 1 (Trivial) | ~3 minutes

Change: Other

Suggested reviewers: shunkica

Merge Risk: ⚪ Minimal · up to 1194f

The per-signature guidance matches how signing and verification retain or clear instance state, so this documentation change is ready to merge.

Architecture Summary

Architecture risk: 🔵 Low · up to 1194f

The change affects 1 system.

Changed systems: README.md

Architecture concerns
No architecture-level concerns identified.

Review details

Systems and components

• observed — README.md (service) was modified; 1 changed file maps to changed impact.

Before / after behavior

• observed — Modified behavior in README.md: Adds a “One instance per signature” section documenting instance state across signing and verification calls: signing uses all accumulated references, checking replaces the references with those in the checked signature, and successful signed-reference results persist until a failed check empties them.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main documentation change: advising users to use one SignedXml instance per signature.
Linked Issues check	✅ Passed	Issue [#409] asks whether callers should reuse SignedXml and whether repeated references require a clearing method or runtime fix. The PR documents one SignedXml instance per signature, including …
Out of Scope Changes check	✅ Passed	The PR changes only README.md. The documentation directly addresses the SignedXml reuse and duplicate-reference behavior in issue [#409]. No unrelated change is shown.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.63%. Comparing base (2a236e8) to head (1194fe1).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #624   +/-   ##
=======================================
  Coverage   83.63%   83.63%           
=======================================
  Files           9        9           
  Lines        1210     1210           
  Branches      307      307           
=======================================
  Hits         1012     1012           
  Misses        119      119           
  Partials       79       79           

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Actionable comments posted: 2

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Inline comments:
In `@README.md`:
- Line 393: Update the README claim about addReference() and computeSignature()
to qualify that checkSignature() replaces the instance’s references, so an
earlier added reference may not be included if verification runs first.
- Line 395: Update the README description of getSignedReferences() and
checkSignature() to state that references are retained only from successful
checks since the most recent unsuccessful validation attempt, which clears
earlier references when validation fails or throws after beginning.

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