test(cache): lock in byte-stability of MCP and protocol payloads

[jhonatanjunio]

Closes #7

Summary

Adds regression-guard tests that lock in byte-stability of every squeez surface that lands in Claude's cacheable prefix. With a warm cache the same session costs ~10× less; a silent drift in one of these payloads can quietly destroy that. Now any breaking change must explicitly break a test.

Audit findings

Surface	Cacheable?	Status
protocol::full_payload()	Yes (tool-result text)	✅ Already stable (two &'static str concatenations)
mcp_server::tools_list_response	Yes (MCP tools layer)	✅ Already stable apart from JSON-RPC id
mcp_server::initialize_response	Yes (MCP handshake)	✅ Already stable apart from id
persona::text(...)	Yes (banner content)	✅ include_str! constants — pointer-identical
wrap header (# squeez […])	No — each call is a fresh tool-result, suffixed not prefixed	n/a, elapsed_ms correctly varies

No code changes were needed — every audited surface is already cache-friendly. This PR is preventive: it ships the tests and doc comments so the next refactor can't silently regress us.

What changed

• tests/test_cache_stability.rs (179 lines) — 7 byte-equality assertions plus a tiny strip_id helper test.
• src/commands/protocol.rs::full_payload — doc comment naming the cache-stability requirement.
• src/commands/mcp_server.rs::tools_list_response and initialize_response — same doc comments referencing the test file.

Test output

running 7 tests
test persona_text_is_identity_stable ... ok
test mcp_initialize_payload_is_stable_modulo_id ... ok
test protocol_payload_constants_are_compile_time_stable ... ok
test strip_id_helper_works ... ok
test protocol_payload_is_byte_stable_across_calls ... ok
test mcp_tools_list_payload_is_stable_modulo_id ... ok
test mcp_protocol_tool_payload_is_stable_across_calls ... ok

test result: ok. 7 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.01s

Out of scope

• Server-side cache regressions (e.g. Claude Code bugs that bypass the cache) — outside our control.
• Banners outside MCP (e.g. init's session-start stdout) — those are message content, not part of the cached prefix; per-session variation is expected.

[jhonatanjunio]

Superseded by upstream PR — moved to claudioemmanuel/squeez. See README of this fork for the upstream link.