[JS][J1] Publish parity matrix and canonical request, URI, and typed-error core

Objective

Turn the JS/Python parity plan into a versioned matrix and implement the smallest runtime-neutral JS core shared by Node and browser builds.

Scope

Capability matrix; canonical URI/request/response serialization; transport-independent typed errors; abort/timeout semantics; runtime feature detection; fixture runner. Do not implement privileged Node credentials, UI components, full Library installation or WASM loading.

Acceptance criteria

• Every matrix row is implemented, blocked with an exact prerequisite, or explicitly out of scope.
• JS passes language-neutral URI, serialization and typed-error fixtures shared with Python/Rust.
• Browser-safe imports do not transitively include Node-only modules.
• Abort, timeout, malformed response and unsupported capability tests pass.
• Public API and compatibility/version policy are documented.

Parent: #74

Tembo execution contract

automation:
  eligible: true
  executor_profile: tembo
  readiness: blocked
  primary_repository: TinyChain-Inc/client
  base_branch: main
  writable_scope:
    - js/src/**
    - js/tests/**
    - js/ROADMAP.md
    - web/tests/**
    - web/package.json
    - web/package-lock.json
  prerequisites:
    - "TinyChain-Inc/tcv2#49 canonical fixtures"
  concurrency_group: js-client-core
  required_commands:
    - "cd web && npm ci"
    - "cd web && npm run test:all"
  external_actions: none
  draft_pr_only: true
  completion_authority: human
  visual_validation:
    required: false
    evidence: []
  budget:
    size: large
    max_revisions: 3
    max_wall_minutes: 120

Begin with the prerequisite TinyChain-Inc/tcv2#49 for canonical fixtures, then inspect js/src/, js/tests/, web/tests/**, and js/ROADMAP.md. Run cd web && npm ci and cd web && npm run test:all after the fixture contract is available. Done means the matrix is resolved, shared fixtures and runtime-safety tests pass, and the public API and version policy are documented.