docs: document y-tag multi-value compatibility contract after #653

[mostronatorcoder]

Context

PR #653 expands y tags from a single token form like ["y", "mostro"] to potentially include platform metadata values (e.g. ["y", "mostro", "<instance>"]).

Problem

External indexers/clients may parse y as a single-value tag and could break if this contract is not documented clearly.

Requested work

• Add compatibility section in docs/changelog describing the y tag contract
• Explicitly state:
  • y[0] remains mostro
  • additional values are optional metadata and must be treated as additive/non-breaking
• Add migration guidance for consumers that currently assume len(y)==1

Acceptance criteria

• Public-facing docs explain the new tag shape with before/after examples
• Changelog/release notes include compatibility note
• No ambiguity about required vs optional y values

Related

• Implemented in Adding instance name to "y" tags #653