Refactor roots, tidy and migration for cohesive ownership

[Yakwilik]

Root resolution, tidy import repairs and migration previously shared mutable resolver and transaction state. Make ownership explicit inside the existing packages: graph resolution returns owned checked evidence, tidy uses focused input/planning/validation units, and read-only file observations are separate from staged writes.

Migration now groups local/Git selection proofs and exposes preflight, authorized dependency verification and final output stages. One candidate operation updates preview and staged bytes/modes together. Named Git migration requests make whole-namespace, retained-root and explicit-root proof intent readable without changing public roots semantics.

The source behavior, CLI/configuration, Go 1.26.6, dependencies and lock/cache formats are preserved. Add three ownership regressions demonstrated red before implementation: reused provider inspection buffers, mutation of returned checked views, and mutation of captured observation bytes.

Validation:

• Full source race suite and vet passed on the completed runtime implementation 265fb529.
• Fresh CLI/MCP with all EASYP variables passed the complete standard v1 suite with -race (102.867s), including SDK compilation, migration, import repair, cold/frozen replay and breaking controls.
• Python gRPC runtime/imported-message scenarios passed (21.024s); fresh terminal wizard and Git migration controls passed.
• Pinned golangci-lint 2.14.0: zero issues. Schema/dev-tool/native example checks passed; no schema or dependency drift.
• Eleven live Git cases attempted: ten passed; gRPC exceeded its 300s get timeout in the parallel run. A fresh isolated gRPC rerun passed get/tidy/generate/cold-frozen/vendor (114.472s), with no source changes. The initial timing cause was not established. The external reported-plugin reproduction was not run without its plugin binary.
• Independent spec and quality gates approved each task; final integration/code review approved with no findings.

Direct state coupling audit: tidy resolver-type references 3→0, resolver-map reads 3→0, transaction storage access outside its implementation 6→0, migration Plan staged-change access 5→0. These are direct reference counts, not a general complexity score.

Stacked on external roots and tidy implementation #236 so reviewers see only this refactor. No merge or release tag is included.