gc: shared delete budget + pagination-robust prefix discovery

[jaredLunde]

Summary

Two independent bugs in the S3 GC path, each with a proof-by-regression-test:

• discover_s3_prefixes truncated at the first list_with_delimiter page. The ObjectStore trait makes no pagination guarantee for list_with_delimiter — it's per-impl. A non-paginating backend silently dropped every export alphabetically past the first page. Switched to a list / list_with_offset walk that jumps past each export's subtree by setting the offset to {name}0 ('/' = 0x2F, '0' = 0x30). O(exports) round-trips on real S3 (start-after is server-side), correct by trait contract on any backend.
• per_prefix_budget = max_deletes / prefixes.len() collapsed to zero. When prefixes outnumber the global delete cap (e.g. 100K+ exports with default max_deletes), GC found dead packs and deleted nothing. Replaced with a single Arc<AtomicUsize> claimed across all prefix tasks via try_claim_delete_slot. Side benefit: budget never gets stranded on prefixes with nothing to delete.

Proofs

Both regression tests were run against the original buggy code to confirm the bug, then against the fix to confirm it passes:

• test_gc_shared_budget_caps_total_deletes_across_prefixes — 3 prefixes × 1 dead pack, max_deletes=2. Old division: 2/3 = 0 → 0 deletes. New shared budget: 2 deletes, 1 survivor.
• test_discover_s3_prefixes_robust_to_non_paginating_backend — uses a new TruncatingDelimiterStore that caps list_with_delimiter at 2 entries with no continuation. Old code: 2 of 5 prefixes returned. New code: all 5.
• test_discover_s3_prefixes_many_exports — sanity check against InMemory with 1500 exports.

Test plan

• cargo test --lib -p glidefs --features test-utils cli::gc — 27/27 passing
• Each new test exercised against the original buggy code path and verified failing

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