docs(wit): freeze WIT clarifications, fix macro nits

[mfw78]

Summary

Stacked fixes from the M1 QA sweep against feat/m1-echo-venue: a WIT freeze
clarification for nexum-value-flow amount encoding and the native-token
variant, a macro correctness fix for venue manifests, and a doc-comment
spelling normalisation, plus documentation of a deliberate charging asymmetry
in the pool router. No wire format, identifier, or behavioural change to the
base set introduced by this stack.

Changes

• docs(wit): mandate minimal-length canonical big-endian encoding for
  asset-amount's amount field and require decoders to compare amounts by
  value, not by byte equality [finding wit: author the nexum:value-flow types package (0.1) with an identifier-hygiene gate #226/amount, minor]
• docs(wit): document native-token(offchain) as representable-but-
  intentionally-invalid; consumers MUST reject that pairing since an
  off-chain settlement domain has no gas token [finding wit: author the nexum:value-flow types package (0.1) with an identifier-hygiene gate #226/native-token,
  minor]
• fix(macros): made the shared missing-[capabilities] error macro-neutral
  so a venue crate is no longer told to use #[nexum_sdk::module]
  [finding sdk: add the #[nexum::venue] proc-macro (adapter cdylib glue + per-component world) #232, nit]
• fix(macros): mirrored synthesize's package-accumulation loop in
  synthesize_venue so a future non-packageless venue capability reaches the
  resolve path (the base package set is unchanged today, since all
  venue-permitted capabilities are currently packageless) [venue-authoring,
  nit]
• docs: normalised authorise/authorisation doc comments to British -ise
  spelling across nexum-intent/types.wit, header.rs, reference.rs, and
  world.rs (recognises); doc comments only, no identifier or API name
  changes [integration-hygiene, nit]
• docs(runtime): documented the deliberate pool submit charge asymmetry in
  submit()'s doc comment: a forwarded submission is charged once the guard
  admits it regardless of venue outcome, while a derive-stage non-decode
  venue error is left uncharged so the caller may retry

Test plan

All five ran inside nix develop --command with --locked and
--config 'build.target-dir="/code/nxm/runtime/target"'
(RUSTFLAGS/RUSTDOCFLAGS=-D warnings, mirroring just ci):

• cargo fmt --all -- --check = PASS
• cargo clippy --workspace --all-targets --all-features -- -D warnings = PASS (exit 0)
• cargo doc --workspace --no-deps (RUSTDOCFLAGS=-D warnings) = PASS (exit 0)
• cargo build --release --target wasm32-wasip2 (example, twap-monitor, ethflow-watcher, price-alert, balance-tracker, stop-loss, flaky-bomb, fuel-bomb, memory-bomb) = PASS (exit 0)
• cargo test --workspace --all-features --no-fail-fast = PASS (exit 0), 42 suites test result: ok, zero failed/ignored-as-failure

Delta sweep (4cff6f2...HEAD): no em-dashes and no #NNN issue references in
.rs/.wit files; commit range 4cff6f2..HEAD carries no attribution
strings; the four new fix commits likewise carry none.

Notes for reviewer

Part of #137.

Stacked on feat/m1-echo-venue; merges bottom-up after the fault train,
retarget to develop then.

[jean-neiverth]

nitpick: crates/nexum-runtime/src/host/pool_router.rs - the new charge-asymmetry doc explains that forwarded submissions are charged once the guard admits them, and decode failures are charged to prevent garbage-feeding. It is silent on whether guard denials consume quota (they do not - the charge happens after the guard). Worth stating explicitly so a reader does not have to trace the control flow to confirm.