fix(transport): make NodeHIDTransport reachable from consumers

[zy0n]

Problem

NodeHIDTransport shipped in the package but no consumer could reach it.

The exports map declared only ".", so the deep path that transport-factory's own
error message directs Node consumers to — dist/core/transport/nodehid-transport.js —
was blocked by Node (ERR_PACKAGE_PATH_NOT_EXPORTED) and TypeScript (TS2307). The
class was not re-exported from the root either.

That made both remedies the factory suggests impossible: you could not import it
directly, and you could not construct one in order to inject it via transportFactory.
The only route left for a Node consumer was reimplementing HWTransport against
node-hid — duplicating a transport this package already ships and tests.

Fix

• ./node subpath export — @railgun-community/ledger-client/node
• root re-export of NodeHIDTransport, alongside WebHIDTransport / WebBLETransport
• @ledgerhq/hw-transport-node-hid declared as an optional peer dependency. It was a
  devDependency only, so consumers were never told to install it, while WebHID and Web BLE
  were already declared this way.
• factory error message and docs/api/transport.md now name the supported import path

Why both a subpath and a root export

The root re-export alone does not unblock a Node consumer. Measured from a real consumer
in bare Node ESM:

SUBPATH OK  -> NodeHIDTransport = function NodeHIDTransport
ROOT FAIL: ERR_MODULE_NOT_FOUND .../@ledgerhq/errors/lib-es/helpers

The root barrel statically pulls @ledgerhq/hw-transport-webhid and
@ledgerhq/hw-transport, which reach @ledgerhq/errors@6.32.0, whose lib-es/index.js
uses an extensionless relative import that Node's ESM resolver rejects. Bundlers resolve
it; bare node does not. This is pre-existing — webhid-transport.js and
ledger-transport-adapter.js each fail identically on their own, independent of this
change.

The subpath sidesteps it: nodehid-transport has no static @ledgerhq imports at all
(only ./types.js, ../errors.js, ./apdu-wire.js). That is also why the root
re-export stays browser-safe — the node-hid dependency is behind a dynamic import
inside connect(), the same shape WebBLETransport already uses.

Verification

• yarn lint, yarn typecheck, yarn build clean; 547 tests / 38 files pass
• packed the tarball and installed it into a clean consumer — nodehid-transport.{js,d.ts}
  present in package/dist/, and import ... from '@railgun-community/ledger-client/node'
  resolves at runtime
• consumer-side TypeScript resolution confirmed under --module nodenext

Follow-up, not in this PR

Making webhid-transport and ledger-transport-adapter lazy would fix the root barrel
under bare Node ESM. That is a behavioral change with real risk and does not belong in a
packaging fix.