fix(transform): flip compressHistory default to false

Round-3 measurement on 2026-05-19 with 128-turn history showed the
feature loses money: 21K chars of text replaced with 140K tokens of
imagery, net -250% savings per request on this traffic shape.

The MARGINAL warning has been in the code comments since the feature
shipped - it was opt-in via --history / COMPRESS_HISTORY=1 originally
for exactly this reason. The recent flag-removal commit flipped it on
as default without re-verifying the savings claim against current
traffic.

The math in baselineCost() correctly reports negative savings when
imgTokensEst > txtReplaced, so the dashboard was honest about the
loss - the bug is shipping the feature on by default before measuring
a positive delta on representative traffic.

Re-enable per-deployment via env var when measurement confirms a
positive saved_pct on the specific shape.

src/core/transform.ts

@@ -124,12 +124,14 @@ const DEFAULTS: Required<TransformOptions> = {
   // `find` over a big tree or `grep -r` can easily exceed this; the paging
   // marker tells the model what was elided. Tuneable per session.
   maxImagesPerToolResult: 10,
-  // Variant C history-image: ON. Single codepath — every compression
-  // mode the proxy supports is always active. Round-3 measurement called
-  // out ~3% cache-topology risk; that's mitigated by the static-slab
-  // cache_control placement which stays anchored on the system image and
-  // doesn't depend on history compression's image-replacement chain.
-  compressHistory: true,
+  // Variant C history-image: OFF. Round-3 spec called this MARGINAL
+  // (~1× per-call) against HIGH cache-topology risk. Live measurement on
+  // 2026-05-19 confirmed the warning: with 128-turn history, replacing
+  // ~21k chars of text with ~140k tokens of imagery LOSES money on every
+  // request. The math in `baselineCost` correctly reports negative savings
+  // when imgTokensEst > txtReplaced. Re-enable per-deployment only after
+  // measuring a positive delta on your specific traffic shape.
+  compressHistory: false,
   historyKeepTail: 4,
   historyMinPrefix: 10,
   // English ~4 chars/tok default (= the CHARS_PER_TOKEN constant declared