source_reference_repoint: a 'retire member' mode on the same token-stream reader

[gunbai-bot]

XL-2 (lively-crane-656). This lands before the filter/any retirement, which will use it.

Why

tools.source_reference_repoint (landed with #13388) REPOINTS a reference from one module to another. Retiring v2.std.algebra filter/any onto the collection roster's callback form (calm-boar-904's ruling B') needs the other operation: DROP a member from every import list naming it from that module. There is nothing to repoint it to, because the bare head resolves through the roster. The ~145 import sites must be rewritten by the generated instrument, never by hand. So this extends the one instrument rather than adding a second tool, which would be a §3 fork.

What

• ImportMemberRetirement { module_path, member }, and the roster source_import_member_retirements. It starts empty; its first rows land with the filter/any PR.
• import_member_retirement_edits: reads the same seed token stream (parsed_import_observation_live) and finds each import <module_path> { ... } statement.
  • Each retired entry is deleted with exactly one adjacent comma, so single-line lists, multi-line lists and trailing commas keep their layout.
  • Overlapping deletions (adjacent retired members) are merged before apply_source_edits, which requires disjoint spans.
  • A statement left empty loses its whole line.
• Comments are not tokens, and a string is one token, so neither is touched. The same name imported from a DIFFERENT module is a different reference and is kept.
• rewritten_source_text = moves, then retirements. The repoint and repoint_pending entries use it, so the walk, idempotence and pending report are shared, not duplicated.

Controls (test.claim.source_reference_repoint_witness_test, supplied text at the instrument's interface)

• { filter, X } → { X }; { X, filter } → { X }.
• An import of only { filter } loses its line.
• A comment, a string, and filter imported from another module are all untouched.
• A multi-line list keeps its layout, including a trailing comma.
• Two adjacent retired members become one deletion.
• A second run is a no-op.

15/15 PASS (8 existing + 7 new): https://app.buildbuddy.io/invocation/a7665d2e-bed8-4f8c-8bd0-ffd710fa0a73

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