fix(compression): record no-op compression runs so Stacked is visible in analytics (#4268)

[diegosouzapw]

Closes #4268

Problem

Stacked (RTK→Caveman) compression is dispatched correctly on the /v1 path, but byMode.stacked.count stays flat and analytics show nothing — making it indistinguishable from "never ran". Root cause (confirmed with the maintainer's own diagnosis): a compression_analytics row is written only on a net-positive saving (chatCore.ts: result.compressed || fallbackApplied || cavemanOutputModeApplied). When a Stacked pipeline runs on already-compact context, both engines no-op → net 0 → no row at all. Ultra's heuristic almost always trims something, so its count climbs — exactly the asymmetry the reporter observed.

Fix (additive, no migration file needed)

• New skip_reason column on compression_analytics, added through the existing ensureCompressionAnalyticsColumns ALTER path (same pattern as the other extended columns).
• chatCore now records attempted-but-no-op runs via a new writeCompressionSkip (tokens_saved = 0, skip_reason: "no_savings"). The else only fires when result.stats is truthy — i.e. the pipeline genuinely ran (the early stats: null returns for empty/disabled bodies are untouched), so we don't record spurious skips.
• Aggregation introduces one successWhere = appendCondition(whereClause, "skip_reason IS NULL") applied to every saving aggregate, so existing totals/averages are byte-identical (no skip rows exist for historical data). Skips are surfaced separately as per-mode skipped, plus totalSkipped and bySkipReason.
• UI (Mode Breakdown) shows · N skipped (no-op), so a mode that only ever no-ops is now visible (count 0, skipped > 0).

Validation (Hard Rule #18 — TDD)

tests/unit/compression/compressionAnalytics.test.ts:

• skip rows are counted separately without polluting saving aggregates (totalRequests/byMode.count/totalTokensSaved exclude them);
• a skip-only mode still appears (count 0, skipped 2, totalSkipped, bySkipReason.no_savings).
• Fail-before proven by reverting the successWhere exclusion → both new tests fail; restored → 17/17 green.

typecheck:core 0 errors, eslint clean, related compression suites green.

Note: active modes that no-op now write one (fire-and-forget, off-hot-path) row per request. That's the point of the transparency ask; net-saving accounting is unchanged.

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