[vfs] Surface natural metadata timestamps

[0xdewy]

Summary

• add optional Entry.modified metadata and teach the mount adapter to expose it through file attrs
• surface natural timestamps for request artifacts, outbox artifacts, wallet EVM outbox actions, DeFi sessions, Polymarket drafts/receipts, Hyperliquid sessions, simulate sessions, watch specs/artifacts, and chain RPC metadata
• make chain head/block/confirmed-tx timestamps best-effort from RPC block timestamps, falling back to the previous synthetic timestamp behavior when unavailable
• expose Base-style unknown typed chain tx/receipt JSON through raw RPC fallback so 0x7e system/deposit tx reads work
• ignore local cache/ and out/ directories

Notes

• Direct VFS reads were manually exercised for chain head/block/tx metadata and existing artifact-backed paths.
• Real Base 0x7e transaction reads were manually exercised through bloom vfs cat using tx 0xc2080cd9bfe974c488e3d78ee60f04d8fbeb93042d668b5dafd16e27179fecc8.
• Mounted stat could not be verified in this environment because kernel mounting requires superuser/passwordless sudo. Tracked separately in Add non-mounted VFS stat or clearer mount permission UX #96.
• Fixes VFS chain tx reads fail on Base system tx type 0x7e #95.

Checklist

• Tests added or updated for behavior changes
• Architecture docs (docs/architecture/) updated if contracts or behavior changed - N/A, metadata timestamps and raw tx fallback use existing VFS entry/read contracts
• Sealed Approval invariants respected (no signing outside a grant or bounded capability, no PRF/grant persistence, execution from sealed bytes) - no signing paths changed
• Agent Documentation updated (crates/bloom-vfs/src/docs/agent-guidance.md and affected Petal READMEs) - N/A, no agent command surface or Petal guidance changed

Tests

• cargo check -p bloom-vfs
• cargo check -p bloom-mount --features mount
• cargo test -p bloom-vfs handlers::chains::tests --lib
• cargo test -p bloom-vfs handlers::outbox::tests --lib
• cargo test -p bloom-vfs handlers::simulate::tests --lib
• cargo test -p bloom-vfs handlers::watch::tests --lib
• cargo test -p bloom-vfs handlers::wallets::tests --lib
• cargo test -p bloom-vfs handlers::requests::tests --lib
• cargo test -p bloom-vfs handlers::defi::tests --lib
• cargo test -p bloom-vfs handlers::polymarket::tests --lib
• cargo test -p bloom-vfs handlers::hyperliquid::tests --lib
• cargo test -p bloom-mount --features mount adapter::tests --lib
• cargo test -p bloom-vfs unknown_typed_tx_receipt_falls_back_to_raw_json -- --nocapture
• cargo test -p bloom-vfs handlers::chains::tests:: -- --nocapture
• cargo test -p bloom-evm
• cargo clippy --workspace --all-targets -- -D warnings
• git diff --check
• Manual: bloom --home /tmp/bloom-base-vfs-test vfs cat /chains/base/tx/0xc2080cd9bfe974c488e3d78ee60f04d8fbeb93042d668b5dafd16e27179fecc8/{full.json,receipt.json,status,block_number,gas_used,logs.json,error.json}