docs(agglayer): sync spec, diagram & comments with #2771

[mmagician]

What

#2771 moved faucet metadata onto the bridge (faucet_metadata_map), removed the faucet's asset_to_origin_asset / get_metadata_hash / get_scale procedures, added the native lock/unlock path, and grew CONFIG_AGG_BRIDGE to 18 felts. Several docs, the spec, and a few comments were left describing the old FPI-based design. This PR brings them in line with the merged code. Docs/comments only - no logic changes.

Changes

• SPEC.md §3.1 (Bridge Account Component):
  • Removed the FPI references in bridge_out / bridge_in::claim (metadata is now read from faucet_metadata_map via bridge_config::get_faucet_conversion_info / get_faucet_scale / get_faucet_metadata_hash).
  • Corrected register_faucet inputs and the [1, is_native, 0, 0] registry value; documented the faucet_metadata_map writes.
  • Added store_faucet_metadata_hash and the internal metadata-reader helper docs, plus a faucet_metadata_map storage row.
  • Documented the native lock/unlock branch on both bridge-out and bridge-in.
• SPEC.md §3.2 (Faucet Account Component): the component now re-exports only mint_and_send + receive_and_burn on top of Ownable2Step + OwnerControlled. Deleted the removed proc subsections and the dead faucet storage slots.
• SPEC.md §4.3 (CONFIG_AGG_BRIDGE): note is 18 felts now - fixed the layout table and consumption description (register_faucet + store_faucet_metadata_hash).
• bridge_out.masm / tests/agglayer/bridge_out.rs: dropped the stale "via FPI" wording.
• diagrams/bridge-out: replaced "FPI: get amount / FPI: get metadata" with bridge-storage reads; regenerated the PNG via make agglayer-spec.

Verification

• cargo build -p miden-agglayer (compiles the edited embedded MASM comments).
• cargo +nightly fmt --all --check and typos clean.
• Repo-wide grep confirms no remaining stale references (removed procs, "via FPI", old storage slots) outside the one sentence that explicitly documents their removal.
• Regenerated bridge-out.png visually verified.

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