[codex] Harden linux-xr canary storage proof

[Jesssullivan]

Summary

• Adds S3/raw-Git upload hardening for the linux-xr canary: .idx files use the large-object chunk profile, streaming upload snapshotting keeps metadata instead of whole chunk payloads, and per-file chunk uploads use bounded fanout via TCFS_UPLOAD_CHUNK_CONCURRENCY.
• Adds a VFS regression proving symlink JSON indices produced by sync push are readable through lazy VFS readdir/getattr/readlink.
• Archives the home-canary-linux-xr-shadow-20260510T201809Z host packet as partial/blocking evidence and updates docs to keep the claim boundary explicit.

Claim Boundary

This PR does not claim full linux-xr parity. The new packet proves isolated shadow inventory/copy, local 85-symlink target preservation, and symlink-enabled push, but honey mounted symlink verification failed and the Linux lifecycle companion failed. Production Finder, M4 mounted reverse read, M8 delete/rename while peer-unsynced, and keep-synced/pin semantics remain open.

Validation

• bash scripts/test-home-canary-linux-xr-shadow.sh
• cargo test -p tcfs-chunks
• cargo test -p tcfs-sync --lib
• cargo test -p tcfs-vfs pushed_symlink_json_index_reads_through_vfs --test vfs_lifecycle_test
• cargo test -p tcfs-vfs --test vfs_lifecycle_test
• cargo test -p tcfs-cli cli_unsync
• bash scripts/test-lazy-hydration-mounted-smoke.sh
• task docs:links
• task lazy:check
• git diff --check
• git diff --cached --check