Stream-id format validator.

Single source of truth for what "a valid stream id" means in reckon-db. Used by reckon_db_streams:append/4 to reject malformed ids at write time so the store never accumulates polluted paths.

Accepted formats

• **User stream:** <prefix>-<hex>, where:
  • <prefix> is [A-Za-z]+ — one or more ASCII letters, no digits, no hyphens, no $.
  • - is a single mandatory separator.
  • <hex> is [A-Fa-f0-9]+ — one or more hex digits, typically a UUIDv7 with dashes stripped.
  Examples: account-018f6a7b8c9d4abc8901234567890abc, order-deadbeef.
• **System stream:** $<namespace>:<name>, where:
  • $ is a mandatory prefix.
  • <namespace> is [a-z][a-z0-9-]* — lowercase identifier, may contain hyphens (e.g. link-sub).
  • : is a single mandatory separator.
  • <name> is [A-Za-z0-9][A-Za-z0-9_.-]* — intentionally human-readable; the whole point of system streams is operational legibility.
  Examples: $link:high-value-orders, $link-sub:revenue.

See guides/system_streams.md for the rationale.

Rejected (returns `{error, Reason}')

• Empty binary — empty
• Not a binary — not_binary
• Anything starting with $ that isn't a well-formed system id — malformed_system_id
• Anything not starting with $ that isn't a well-formed user id — malformed_user_id

Rejected examples: test$basic-stream (mid-string $ in a user id), partition$XYZ (ditto), $weird (no :), MYAPP-abc (prefix is fine but abc starts compliant — and xyz isn't hex; g would be rejected), account- (empty hex), -deadbeef (empty prefix).