Collapse validate() into a single generalised method threading mode and options #29

Observation

Every per-kind shape validator now handles the same three modes (see validateValue in src/value.core.ts):

1. state — validate an instance value against the shape
2. state against model — validate a retrieval result narrowed by a projection template
3. model — validate a retrieval template (placeholder)

The mode is currently threaded piecemeal: validateValue takes a model?: boolean flag and forks per kind
(validateText/validateLocale, validateResource/validateTemplate, etc.), while the public validate() in
src/index.ts carries a four-branch dispatch (union, non-resource value, model === true template, keyed-model
result, plain resource) with a separate seal/idempotence check in each branch.

Idea

Generalise the mode into a single parameter carried end to end:

• Pass model: boolean | Template (or a richer descriptor X) uniformly, matching the union already declared on the
  merged validate() options (src/index.ts model?: boolean | Template).
• Thread the remaining options (entry, plain, depth, limit) through validateValue
  (src/value.core.ts L511-554) so each leaf validator receives the full context rather than re-deriving it at the
  index.ts layer.
• With the mode and options unified, the four validate() overload bodies (src/index.ts L179-573) should collapse
  to a single generalised method dispatching once on shape kind and mode.

Scope / risks

• Public validate() overload signatures stay as-is (state / result / template / value); only the shared
  implementation body collapses.
• Idempotence sealing currently differs per branch (Validated seal payload varies by mode); a unified path must
  preserve those distinctions.
• Behaviour-preserving refactor: existing test suite must stay green throughout.