feat(mix): support blur tileMode across core and protocol

[leoafarias]

Description

BlurModifier now accepts a tileMode, and the protocol encodes and decodes that same field. Keeping the core model and protocol inventory together lets the schema guard pass for the new two-field blur representation.

Changes

• Add tileMode to the core modifier, generated data code, and configuration API, with focused tests and reference documentation.
• Encode, decode, and walk tileMode in mix_protocol; update its field inventory, wire contract, schema, and codec tests.
• Refresh the downstream chart protocol schema generated from the Mix vocabulary.

Review Checklist

• Testing: Focused Mix blur tests, protocol codec tests, protocol inventory check, and melos run ci passed locally on Flutter 3.44.0.
• Breaking Changes: No existing blur call site needs to change; the default remains TileMode.clamp.
• Documentation Updates: Wire contract and modifier reference updated.
• Website Updates: No website change is needed.

Additional Information

#1081 is the CI prerequisite. Its in-repo checks allow this core-and-protocol change to land together; its hosted release gate validates both the minimum and latest Mix version before dependent publishing. #1072 currently targets main because GitHub prevents changing the base of a PR with downstream stack branches. Once #1081 lands, its CI commits will disappear from this PR's diff.

Published Mix 2.2.0 does not contain this API. Before publishing a new mix_protocol, release a Mix version containing tileMode, raise the protocol's mix lower bound to that actual published version, and run the hosted release gate. No future version is assumed here.

[tilucasoli]

Closing to re-create as a proper stacked PR based on feat/blur-modifier-tile-mode-mix (#1077). Superseded by the new PR below.