fix(server): hold streamed section titles with content

[rynfar]

Streamed Markdown headings and standalone bold section titles could appear by themselves while the following paragraph, list, or code block was still arriving. Hold those titles until a safe boundary includes content below them. An unindented heading still releases the completed paragraph above it.

Selectively adapts T3 a36af0637a63ebf316beb5db5670be226df01cbc (#13504) to Pylon's existing natural-boundary splitter. Existing final-message flush and bounded-buffer behavior remain in effect. No wire contract or client rendering changes.

Verification: vp test run apps/server/src/orchestration/Layers/ProviderRuntimeIngestion.test.ts (135 passed); pnpm --filter t3 typecheck; scoped vp lint (existing unused ProjectId import warning); scoped vp fmt --check; git diff --check.

Model: GPT-6 Sol (low reasoning), overseen by Codex. Harness: Pylon Codex.

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1 Skipped Deployment

Project	Deployment	Actions	Updated
pylon-marketing	Ignored	Preview	Sep 27, 2026 5:32am UTC

[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	14.0 KiB	14.0 KiB	−10 B (−0.1%)	15.1 KiB	✅
Codex	Thread snapshot wire	7.2 KiB	7.3 KiB	+16 B (+0.2%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.7 KiB	6.7 KiB	−26 B (−0.4%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	58.0 KiB	58.0 KiB	0 B (0.0%)	66.4 KiB	✅
Codex	Live turn messages	9	9	0 (0.0%)	21	✅
Claude	Total thread wire	14.0 KiB	14.0 KiB	−38 B (−0.3%)	15.1 KiB	✅
Claude	Thread snapshot wire	7.3 KiB	7.3 KiB	−5 B (−0.1%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	6.8 KiB	6.7 KiB	−33 B (−0.5%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	58.9 KiB	58.9 KiB	−44 B (−0.1%)	66.4 KiB	✅
Claude	Live turn messages	10	9	−1 (−10.0%)	21	✅

Baseline: 324d708 · PR result: 00a50ea · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 115.7 KiB
• Claude decoded thread snapshot: 116.4 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.