[P1] Align PRD treasury wording with current operator-direct model

[ametel01]

Problem

Most treasury/signer separation language is already aligned in the live docs (README.md, docs/spec.md), and contract behavior already routes payments directly to immutable operator private balance.

Remaining mismatch is in PRD wording that can still imply unavailable treasury controls (e.g. moving accumulated user-paid tokens from contract balance to a designated treasury address).

Also, original validation scope referenced contracts/credit_fpc/src/main.nr, which no longer exists.

Scope

Narrow this task to docs alignment for current Alpha behavior:

• Update local-docs/fpc-prd.md to explicitly reflect current model:
  • fees are paid user private -> operator private directly
  • no on-chain treasury sweep from contract token balance
  • signer/revenue destination are the same operator identity in Alpha
• Remove stale references to removed contract variants/paths in this issue and related wording.
• Optional: add a brief cross-reference note from PRD section to docs/spec.md (limitations/security) for canonical behavior.

Files to change

• local-docs/fpc-prd.md
• (optional) small cross-reference touch-up in README.md or docs/spec.md

Validation

• Cross-check against contracts/fpc/src/main.nr transfer destination and immutable config model.
• Confirm no remaining PRD language implies unavailable on-chain treasury controls for current Alpha.

Acceptance Criteria

• PRD text no longer implies unsupported treasury sweep/admin controls.
• Alpha separation model is explicit: no signer/treasury split on-chain in current design.
• Issue/docs scope no longer references removed credit_fpc path.

Depends on

• [P0] ADR: lock Alpha asset model (multi-asset contract vs one-FPC-per-asset) #132

[ametel01]

PRD updated — local-docs/fpc-prd.md patched

The stale treasury-sweep language has been removed and replaced with an accurate description of the Alpha model:

Before:

Ability to transfer accumulated user-paid tokens to a designated address (so the operator can move them to an account that can bridge back to L1 for treasury management). This doesn't need to be automated for MVP, just possible.

After:

Revenue model: fees flow directly from user private balance to operator private balance at payment time. There is no on-chain treasury sweep or accumulated contract balance to drain — the operator receives tokens directly and manages them off-chain. In Alpha, the signer/revenue destination is the same operator identity (no signer/treasury split on-chain).

This aligns with contracts/fpc/src/main.nr behavior and docs/spec.md. No remaining PRD language implies unavailable treasury controls. Ready to close once PR is merged.