Assembles the st-1g1n, st-5577, st-mvor, st-pnr0, st-q6k2, st-u9l6 and
st-vfmb fragments into a 2.5.0 section and deletes them, and moves the
README requirement to ~> 2.5.

ADR-0066's SemVer commitment makes this a minor release. The public
surface only grows: `Statifier.Invoke.Answer` and its `caller_context:`
option, `caller_context` on the invoke and cancel_invoke effects and
their telemetry metadata, `donedata_error` on the done effect and its
trace, and the `conds_evaluated` trace effect and event. Nothing public
is removed or narrowed. The one signature change - `Selection`'s
`select_transitions/2` and `select_eventless_transitions/1` returning a
third element - is interpreter-internal and outside the telemetry
contract, so it is not a public break. `Session.Recording`'s format
version reaches 5 and still decodes every earlier blob, defaulting the
new fields. The behavior changes are an `<assign>` carrying both `expr`
and markup children now failing validation instead of lowering, and
`done_invocation/3` and `failed_invocation/3` stamping the arming
event's `caller_context` where they left `nil`.

Prep only, per campaign-027 ruling R27-13: the tag and the Hex publish
stay with the operator.

Refs: st-4qct