refactor(debug-files): share directory scan / empty-input policy / upload runner across symbol types #71

Follow-up to #69 / #70 (ELF now accepts directories).

Context

Every debug-files upload --type … flow has its own directory walker (WalkDir in several places in src/cli/debug_files.rs) and, for several types, its own identify → dedup → pack → register → PUT loop. Review of #70 turned up policy details that are easy to get wrong per walker, and the other types may not share the fixes:

• symlink policy (don't descend directory symlinks; read file symlinks)
• unreadable root must fail; unreadable subtree warns
• empty directory = configuration mistake (exit 10) vs silent exit 0; --allow-empty handling (ELF currently rejects the flag)
• deterministic ordering (stable "first wins" tie-breaks), --extension matching, Windows path normalisation

Proposed (two steps)

1. Audit + shared walker. Document how each type's walker differs today, pin the current exit codes/behaviour with tests, then extract one scanner (name match incl. --extension, symlink policy, ordering, unreadable-root rule) and one empty-input policy. Behaviour-preserving; any exit-code change called out in the changelog (exit codes are a public contract).
2. Optional: generic upload runner for the identify → keep-best-per-identity → bounded-concurrency pack/register/PUT pattern (ELF, Rust-ELF, PDB, dSYM, source maps), only if step 1 shows enough duplication to justify the risk.

Non-goals

Per-format identification (src/symbols/*) and wire shapes stay as-is; zip input stays an ELF-only adapter.

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