Make font changes target layers by id

[kostyafarber]

Summary

• Add an explicit GlyphLayerCreated font change for layer row creation.
• Change existing layer edit changes to carry LayerId directly instead of GlyphLayerChangeTarget.
• Update shift-store to persist layer rows by the real LayerId and reject layer edits when the row is missing.
• Simplify shift-workspace change construction now that layer edits no longer need glyph/source/name target hydration.

Checks

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace
• commit hook: cargo check/tests, cargo fmt, cargo clippy