docs(compression): align guide claims with source

[woodsonl]

Summary

• A source check of docs/compression/COMPRESSION_GUIDE.md found statements the code does not bear out. This PR corrects the Caveman Output Mode, Output Styles, Tool Result Compression and language-selection passages against the current sources, and fixes the contradictions a verification sweep found in the surrounding sections (Ultra, Aggressive, Cache-Aware, Progressive Aging, Stacked Pipeline, Combo Overrides, RTK filter count, routing-combo override list).

Main corrections:

• Caveman Output Mode: requests get the instructions through applyOutputStyles() after chatCore.ts resolves the selection with the back-compat shim; outputMode.ts holds the texts, the bypass and the placement helper, and its applyCavemanOutputMode() has no production caller. The example prompt that matches no instruction text is replaced with the actual English full text. The shim maps cavemanOutputMode to terse-prose only while outputStyles is empty, and input compression still runs after injection.
• How injection works: the idempotency check runs before the content bypass; the order-sensitive bypass is a step word followed within 240 characters by an action word, not a keyword list; the no-messages path consults neither the bypass nor the top-level system field, skips an already-marked instructions field as already_applied, and overwrites only non-string instructions/input values.
• Language selection: autoDetect samples only the messages array, so Responses bodies (input) fall back to defaultLanguage; the detector returns a fixed language set and never defaultLanguage on zero hits; vi is never detected; a compression combo forces languageConfig.enabled on and derives defaultLanguage from the combo's language packs.
• Tool Result Compression: the five strategies are listed with their actual triggers (indentation-tolerant code keywords, the path:line: shape a timestamp line also matches, ANSI CSI or $ + whitespace, JSON start/parse conditions, case-insensitive error substrings), plus the {…N keys} placeholder, the error-elision marker's 14-line floor, and the callers' saved <= 0 guard.
• Ultra Mode: an independent score-based prose pruner with structure preservation and an optional SLM tier. It does not include Aggressive mode's features, thin code blocks, or binary-search truncate (no such code exists).
• Aggressive Mode: tool outputs are truncated or elided, not summarized.
• Cache-Aware: a caching provider alone triggers the downgrade (no cache_control markers required, and markers alone never trigger it); the never-consumed deterministic-transformations step is dropped.
• Progressive Aging: documents the shipped thresholds (last 2 turns verbatim, caveman at distance 3, summarized from distance 4, nothing dropped), the exemptions (system prompts, already-aged messages, the latest user message), and that it runs in aggressive mode only, not ultra.
• Stacked Pipeline: numbers composed from the guide's own math (46% Caveman step, ~89% average); configuration points at the stackedPipeline setting or a named compression combo, not an auto-combo modePack.
• Combo Overrides: compressionOverride is a plain mode string; caching providers are handled by the always-on cache-aware adjustment (there is no selectable cache-aware mode); weights/modePack sit at the top level of combo config.
• RTK: 55 built-in filters, not 49. Configuration: the routing-combo override dropdown also offers Codex Responses.

Related Issues

• Related to docs(compression): match caveman output mode notes to code #15297 (adjacent sections of the same guide; based on its head to avoid conflicts)

Validation

• Change type: other (docs-only)
• Focused checks from the golden path: npm run check:docs-all (includes the fabricated-docs gate) — passes
• npm run lint not applicable to a markdown-only diff; prettier applied
• Reconciled with the current active release base release/v3.8.52

Every changed passage was verified claim-by-claim against the source, with evidence recorded per claim (file, line, quote).

⚠️ base-red inherited: #15306 (release/v3.8.52 was red before this branch; the inherited failures are not from this change)

Note: the 66 i18n mirrors of this guide wait for the next refresh.

Tests Added Or Updated

• No production code changed. Docs-only diff, so no tests added.

Coverage Notes

• No code touched; coverage unaffected.

Reviewer Notes

• Each bullet above names the source file that pins the new wording, if you want to spot-check.
• The i18n mirrors under src/i18n (66 locales) are refreshed in bulk and intentionally not touched here.