python: add Lightning reference implementation under bip122/exact

[Bortlesboat]

Description

Adds a Python Lightning reference implementation under python/x402/mechanisms/bip122/exact/.

This keeps the PR scoped to mechanism behavior:

• uses the existing Bitcoin CAIP-2 network identifiers and extra.paymentMethod = "lightning"
• adds LightningPayer / LightningReceiver adapter protocols for client and receiver integrations
• generates BOLT11 invoice-backed payment requirements on the server side
• verifies paid invoices and returns payment_hash from settle() as the settlement identifier
• treats sats as the default Money input and converts to msats internally, with explicit helper functions for sats <-> msats
• adds duplicate-settlement protection keyed by payment_hash
• keeps the core dependency surface narrow by only adding bolt11
• adds focused unit coverage plus a sync end-to-end roundtrip through x402ClientSync, x402ResourceServerSync, and x402FacilitatorSync

This intentionally does not add operator-facing audit or receipt fields like authorized_by to the Lightning settlement response. That is broader x402 workflow metadata and should be handled separately from the Lightning mechanism API.

Closes #1857.

Tests

• cd python/x402 && ./.venv/Scripts/python.exe -m ruff format mechanisms/bip122 tests/unit/mechanisms/bip122
• cd python && ./x402/.venv/Scripts/python.exe -m ruff check x402/mechanisms/bip122 x402/tests/unit/mechanisms/bip122
• cd python && ./x402/.venv/Scripts/python.exe -m pytest x402/tests/unit/mechanisms/bip122 -q
• cd python && ./x402/.venv/Scripts/python.exe -m pytest x402/tests/unit/mechanisms -q
• cd python && ./x402/.venv/Scripts/python.exe -m pytest x402/tests/unit -q
• cd python && ./x402/.venv/Scripts/python.exe -m pytest x402/tests -q

Checklist

• I have formatted and linted my code
• All new and existing tests pass
• My commits are signed (required for merge) -- you may need to rebase if you initially pushed unsigned commits
• I added a changelog fragment for user-facing changes

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 0 Sum 1

[vercel]

@Bortlesboat is attempting to deploy a commit to the Coinbase Team on Vercel.

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[Bortlesboat]

Added compatibility tests against Alby's live Lightning facilitator (x402.albylabs.com/supported). All 7 tests pass — confirms our bip122/exact mechanism is wire-compatible:

• SupportedResponse schema parses correctly
• Network matches: bip122:000000000019d6689c085ae165831e93 (mainnet)
• Scheme: exact, version: 2
• Signers map includes bip122:*

39 total tests passing (32 unit + 7 facilitator compat).