Lock in dependency semantics and redesign parser execution context

[dahlia]

Summary

This issue covers two closely related foundations for the #750 redesign:

• locking in the target dependency semantics with a dedicated regression matrix
• redesigning the parser execution model so parser-local state is separated from shared execution context

These two belong together because the execution-context redesign should be driven by explicitly agreed semantics, not by the incidental shape of the current implementation.

Scope

This issue should cover:

• adding a regression matrix for dependency behavior across top-level parsing and shared-buffer constructs
• locking in expected behavior for derive() and deriveFrom() in parse and suggest paths
• covering wrapper combinations such as optional(), withDefault(), and map()
• covering external resolution paths such as bindConfig(), bindEnv(), and prompt()
• redesigning ParserContext so it can distinguish parser-local frame data from shared execution context
• updating the Parser contract so complete() and shouldDeferCompletion() can receive execution context directly

Non-goals

This issue should not yet:

• introduce the new dependency runtime
• migrate object(), tuple(), concat(), or merge()
• remove the old dependency-state protocol

Completion criteria

This issue is complete when:

• the target dependency semantics are covered by a dedicated regression matrix
• the new execution-context direction is encoded in internal types/interfaces
• the parser contract is updated so later runtime work can build on it

Parent issue

Part of #750

[dahlia]

Recommended starting point

Start by turning the target semantics into explicit tests before changing any contracts. The safest order here is:

• write or expand the regression matrix first
• make sure the tests describe the target semantics, not incidental current internals
• only then change ParserContext, complete(), and shouldDeferCompletion()

The parent design discussion for this issue is in:

• Decouple dependency-system internals from option()/argument() and modifier implementations #750 (comment)
• Decouple dependency-system internals from option()/argument() and modifier implementations #750 (comment)

Key files

• packages/core/src/parser.ts
• packages/core/src/dependency.test.ts
• packages/core/src/constructs.test.ts
• any test files covering top-level parse/suggest behavior if additional coverage is needed

Invariants to preserve

• The regression matrix should cover top-level and construct-owned dependency behavior.
• Parse and suggest paths should both be represented.
• Sync and async variants should be kept aligned as much as possible.
• This issue should establish the new execution-context contract, but should not yet introduce the new dependency runtime.

Known traps

• Do not let the tests merely encode current implementation quirks if those quirks are exactly what #750 intends to remove.
• complete() and shouldDeferCompletion() should move toward execution-context-aware signatures, but this issue should avoid mixing in runtime migration logic at the same time.
• Keep parser-local state and shared execution context conceptually separate from the start, otherwise later issues will inherit the same ambiguity in a new form.