fix(lexical): carry text artifact across added and removed files

[devin-ai-integration]

Summary

• An edit that adds, removes, or renames a file now carries the parent lexical text artifact and re-encodes only the affected files. Previously, it fell back to a cold build of the whole corpus.
• The carried artifact remains byte-identical to a cold build of the same tree.

Fixes #2906
Fixes #2880

Cause

The earlier carry fixes handled only edits that kept the same file roster. changed_files_since returned None when the file count or any file key differed, so adding or removing a file caused a full re-encode. Under a 6 GiB cap, a full-clone edit then rebuilt the whole corpus.

Changes

• VerifiedSealedLexicalPageSourceV1::file_replacements_since replaces changed_files_since. It verifies that both sources cut pages the same way, then calls the pure file_replacement_runs function. That function walks both rosters by logical path. Paths strictly ascend in build order. It returns SealedLexicalFileReplacementV1 runs from parent_files to files, and either side of a run can be empty. It returns None when page-cut inputs differ or paths do not strictly ascend. In those cases, the build remains cold.
• partitioned_codec: replaces the roster comparison with the file_keys() and same_segment() primitives.
• increment.rs: stage_replaced_files stages each run's successor files at the cold position and verifies that the runs ascend together. rewrite_page_rows now also shifts source_pages.file_ordinal by the net file growth of earlier runs. Document, page, and clone shifting is unchanged and operates per run.
• serving.rs: the existing 1/8 cold-fallback threshold now counts replaced files as max(parent, successor) for each run.

Test plan

• New behavior test an_added_or_removed_file_carries_the_parent_text_artifact_byte_identical_to_a_cold_build. It hides every parent segment while the successor builds, so a cold fallback cannot finish. The test then requires the carried bytes to equal a cold build across six rounds: add a file, remove a file, replace a file with its neighbour alongside a body edit, rename a file, add a new first file, and remove the last file.
  • Without this change, it fails with Contract("sealed generation segment is unavailable ..."). With this change, bash scripts/require-exact-test.sh cargo test -p tracedecay-code-index-runtime --lib code_index_scheduler::tests::text_artifact_carry_tests::an_added_or_removed_file_carries_the_parent_text_artifact_byte_identical_to_a_cold_build -- --exact passes with 1 test passed.
• New unit test file_replacement_runs_align_rosters_by_path checks exact runs against literal values. Cases cover no change, a single edit, additions and removals at both ends of the roster, an addition next to a removal as one run, an edit that extends a preceding replacement, every file replaced, and every file removed. bash scripts/require-exact-test.sh cargo test -p tracedecay-code-index --lib production::lexical_page_source::lexical_page_source_tests::file_replacement_runs_align_rosters_by_path -- --exact passed with 1 test passed.
• cargo test -p tracedecay-code-index-runtime --lib -- text_artifact (26 passed), cargo test -p tracedecay-query --lib -- lexical::projection::artifact (84 passed), cargo test -p tracedecay-code-index --lib -- lexical (14 passed)
• cargo fmt --all -- --check; cargo clippy -p tracedecay-code-index -p tracedecay-query -p tracedecay-code-index-runtime --all-targets -- -D warnings is clean
• cargo nextest run --workspace --no-fail-fast (left to CI)

Checklist

• CHANGELOG.md updated (release-please generates it from the commit)
• No secrets, credentials, or .env files included
• Breaking changes documented: none. changed_files_since was used only inside the workspace.

Link to Devin session: https://app.devin.ai/sessions/b676368291d446d1b33cd48d4dd5afad
Open in Devin Desktop: https://app.devin.ai/desktop/session/b676368291d446d1b33cd48d4dd5afad?variant=devin
Requested by: @ScriptedAlchemy

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.