Support u16 memory IDs on the 0.12 line

[humandebri]

Summary

• keep this branch intentionally based on stable-fs v0.12.0 and keep the stable-fs crate version at 0.12.0
• switch StableStorage memory index ranges to u16 and allow IDs up to the highest non-reserved u16 MemoryId
• use humandebri/stable-structures at fixed rev a24a7d7572e36eda104abd8280946aeb7ac4e060 so downstream stable-fs users get the u16 MemoryId API without a separate patch section
• update tests to target the current memory manager layout instead of legacy v0.4 memory fixtures

Important note

This PR is intentionally for the stable-fs 0.12 line. It is not intended to merge directly into current main without retargeting or rebasing, because current main is v0.13.0.

Tests

• cargo check -p stable-fs
• cargo check --workspace
• cargo test -p stable-fs
• cargo tree -p stable-fs -i ic-stable-structures