feat(tx-manifest): add smplx transaction engine

[lukachi]

Stack

PR 3 of 9 in the manifest release train. This is stacked on #11; review only the delta from feature/tx-manifest-review.

What this slice delivers

• turns the canonical explicit p2pk Pay action into a deterministic transaction plan
• selects wallet inputs deterministically and preserves exact bigint amounts
• pins the smplx submodule at 8f0215c and integrates its real SDK 0.0.10 WASM package
• adds the real smplx covenant compiler/loader and transaction-builder adapter
• assembles the reviewed inputs, outputs, fee rate, and caller-supplied change script
• leaves finalization behind an injected, caller-owned finalizer seam
• wires smplx into extension, deploy, Storybook, formatting, lint, and focused CI paths

Deliberate boundaries

This slice does not own wallet credentials, mnemonic access, signing, confidential outputs, covenant-spend inputs, broadcasting, or UI activation. Receive remains an explicit refusal when it reaches the unsupported covenant-input amount reference. Those capabilities arrive in later stacked PRs.

Verification

Verified from a clean detached worktree at this commit with LWK 693c4fb, smplx 8f0215c, and the generated smplx SDK 0.0.10 package:

• frozen install passed
• package TypeScript check passed
• strict TypeScript check for every smplx adapter and test passed
• focused tx-manifest and real-WASM suite: 96 passed, 0 failed
• repository formatting passed
• repository lint exited zero with only the existing warnings

The repository-wide root app typecheck is not used as this slice's gate: a clean frozen install currently exposes pre-existing duplicate React/Zod type-family failures in unrelated UI code. The focused workflow checks the entire surface owned here, and restoring the cumulative full-repository gate is tracked for the final activation/release slice.