[v0.17 CAP-1c] Text lanes bound files by a compile-time constant, not the active source policy

[TheGreenCedar]

Found by adversarial review of #1821, and confirmed against the tree.

The gap

The indexer admits a file by self.source_file_byte_cap, taken from
SourceIndexPolicy.byte_cap, which source_index_policy_from_env_value
(codestory-cli/src/config.rs:73-78) sets from
CODESTORY_INDEX_SOURCE_FILE_BYTE_CAP with no upper clamp — only > 0.

Three per-file text bounds are compile-time constants:

Lane	Constant	Checked at
lexical FTS	MAX_FILE_BYTES	codestory-retrieval/src/lexical_index.rs:970
repo-text scan	REPO_TEXT_MAX_FILE_BYTES	codestory-runtime/src/repo_text.rs:90,109
semantic doc bodies	SEMANTIC_FILE_TEXT_MAX_BYTES	codestory-runtime/src/support.rs:389

Set CODESTORY_INDEX_SOURCE_FILE_BYTE_CAP=4000000 and a 3 MB Rust file is
indexed with full symbols, nodes, and dense anchors, while all three lanes drop
it. There is no error row and no retry_required; derive_degraded_mode
discards the component reason and strict readiness still reports Full. The
user gets a file indexed_files calls complete that search cannot find.

What #1821 did and did not do

#1821 made the three constants derive from DEFAULT_SOURCE_FILE_BYTE_CAP and
added a compile-time assertion to each. That closes the default configuration
and stops a future default bump from silently stranding a band of files — which
is what would have happened when the cap went to 2 MB.

It does not close this, and the assertions cannot: they compare two
compile-time constants and are blind to a runtime override. code-review-2026-08.md
has been corrected to say so rather than retiring CR-068's oversized half.

Done when

• The three lanes bound files by the active SourceIndexPolicy, not by a
  constant
• A test sets CODESTORY_INDEX_SOURCE_FILE_BYTE_CAP above the default,
  indexes a file in the widened band, and asserts it is lexically
  recallable — a constant-comparison assertion is not sufficient, because
  it passes when a lane is bounded by something nobody listed
• Or, if plumbing is rejected: the env override is clamped to the lane
  bound, and the setting's registry description says the cap cannot be
  raised past it

Note

This is a pre-existing gap — CR-068 described it before #1821 — but it is worth
its own issue now because the surrounding claim changed and the residual would
otherwise be invisible.