fix: preserve AI attribution when human edits file after AI checkpoint (fixes #1444)

[IanMcNelly]

Problem

When a human modifies a file after an AI checkpoint fires but before committing — without a known_human checkpoint being triggered — AI attribution for the AI's own lines was lost (shown as 100% untracked).
Reported in #1444.

Root cause

The carryover snapshot stays stale (holds the AI checkpoint version). An overlap filter in the attribution loop was then stripping the human-modified position from unstaged_hunks because it also appeared in
committed_hunks — incorrectly crediting that committed line to AI. For the modify case (human replaces an AI-written line), this meant the human's content was attributed to AI.

Fix

virtual_attribution.rs: Skip the committed-vs-unstaged overlap filter when a carryover snapshot is present. With a snapshot, Replace-type unstaged lines represent content divergence between the AI
checkpoint and the commit — they should flow to INITIAL, not be silently re-attributed.

attribution_recovery.rs / post_commit.rs: Replace the content-based guard in recover_adjacent_edges with a displaced_to_initial_lines set (intersection of initial_attributions and
recovery_hunks). This blocks edge recovery from re-claiming lines explicitly sent to INITIAL, while preserving edge recovery for genuinely adjacent uncheckpointed inserts/appends.

Tests

Added tests/integration/human_edit_after_ai_checkpoint.rs with four regression tests covering:

• Human appends lines after AI block (edge recovery attributes adjacent lines to AI — existing product behavior)
• Human modifies a line within the AI range (the core bug — now correctly unattributed)
• Human inserts lines before AI block (edge recovery attributes adjacent lines to AI)
• Partial staging guard: real uncommitted lines still defer to INITIAL

All existing tests pass including the delayed_checkout/switch_merge_trace_replay tests.

Fixes #1444

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