spec/validate.py accepts a tree source that carry.py refuses, comparing "." as a raw string #1445

Found while fixing #1443 (2026-09-08). A tree source spelled "./" or ".///." passes spec/validate.py and then fails in scripts/carry.py, so a manifest author gets a green validator and a failing carrier.

The mechanism

spec/files.schema.json constrains a tree source as:

"source": { "type": "string", "minLength": 1, "not": { "const": "." } }

That forbids "" and "." and nothing else, so "./" and ".///." are schema-valid.

spec/validate.py adds two of its own rules, and both let those through:

• the root check is value == ".", a raw-string compare, so "./" is not equal to "." and passes
• (ROOT / source).is_dir() is true for "./", since that resolves to the repository root

So validate.py reports green while its own error text for the rule says a source "must be below the repository root".

scripts/carry.py refuses both, because relative_root compares PurePosixPath objects rather than strings, and PurePosixPath("./") == PurePosixPath(".").

Why it matters now

build_dist.py grew its own guard in #1443, which refuses any tree source reducing to the repository root by its own name. That closes the hazard for build_dist.py's consumer only. The validator is still the gate a manifest author runs first, and it is the one that should have caught it.

Suggested shape

Compare normalized paths rather than raw strings in validate.py's tree-source rule, the way carry.py already does. build_dist.py's tree_source() is a third implementation of the same reduction, so this is also a candidate for one shared helper rather than three.

Not a live defect today

spec/files.json declares one tree, source: ".github/skills", so nothing in the fleet currently trips this.