fix(plan): perbaiki bug entropy gate dan fixture test squash.rs

Ditemukan implementer Task 4 sebelum commit apapun (BLOCKED, bukan
kode salah): entropi Shannon mentah per-karakter tidak membedakan
prosa dari identifier acak — prosa berulang skor ~3.89 bit/char,
lebih tinggi dari UUID (~3.39). Tambah syarat "tanpa spasi" sebelum
cek entropi (meniru pre-filter headroom sendiri), turunkan ambang ke
3.0 pada skala mentah. Fixture test array JSON juga diperbesar
(repeat 5 -> 8) karena sebelumnya tidak pernah melewati SQUASH_FLOOR_BYTES
yang diasumsikan test itu sendiri.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docs/superpowers/specs/2026-07-16-context-compaction-overhaul-design.md

@@ -148,12 +148,18 @@ bytes pass through unchanged (compression only pays off on large output, and tou
 results risks losing detail with no token benefit). Above the floor, dispatch by content
 shape:
 
-- `squash_json(&str) -> String` — hand-rolled JSON tokenizer; structural tokens (keys,
-  brackets, colons, commas, booleans, null) always kept; string values kept if ≤20 chars or
-  high-entropy (Shannon entropy ≥0.85 bits/char, catches UUIDs/hashes/paths — same threshold
-  headroom uses), otherwise replaced with `"…"` in place; array elements past the first 3
-  compressed harder (values elided regardless of length/entropy). Applied when
-  `serde_json::from_str` on the output succeeds.
+- `squash_json(&str) -> String` — walks a parsed `serde_json::Value` (not a hand-rolled
+  tokenizer — `serde_json` already handles escaping/nesting correctly, reusing it is simpler
+  and more robust); structural tokens (keys, brackets, colons, commas, booleans, null) always
+  kept; string values kept if ≤20 chars or "identifier-shaped" (no internal whitespace *and*
+  Shannon entropy ≥3.0 bits/char — catches UUIDs/hashes/paths), otherwise replaced with `"…"`
+  in place; array elements past the first 3 compressed harder (values elided regardless of
+  length/entropy). Applied when `serde_json::from_str` on the output succeeds. The
+  no-whitespace pre-filter matters: raw per-character entropy alone doesn't separate prose
+  from identifiers — repeated English prose measures ~3.89 bits/char, higher than a UUID's
+  ~3.39 — because prose also draws from a wide character set. headroom's own entropy gate is
+  "cheaply pre-filtered by 'no spaces'" before scoring for the same reason; multi-word values
+  never reach the entropy check at all under this rule.
 - `squash_log(&str) -> String` — line classifier (error/fail/warn/info/debug/trace by
   keyword + stack-trace-frame detection) → score
   (`level_score {1.0 error/fail, 0.5 warn, 0.1 info, 0.05 debug/trace} + 0.3 if