types: settle a shared field name once the binding is inferred

[simontreanor]

From the other repo's feedback that #37 did not fully remove the field-name problem. Testing it splits the report in two, and one half is fixable.

Map.tryFind is not the culprit. When the map's value type is pinned, the payload already resolves:

let board = Map.ofList [(1, Placed { … })]
match Map.tryFind k board:
  case Some p: p.letter        # ok today

It fails only when the map itself arrives unpinned, which is the same root cause as a bare lambda parameter — nothing determines the type — rather than anything about tryFind or Option payloads. There's a test for this so it stays true.

The fixable shape is a base that a later statement pins down:

let describe c =
    let l = c.letter        # rejected before: `c` unknown *here*
    let flag = c.blank      # …but `blank` determines `c`
    l

HM knows the answer by the end of the binding; #37 just asked too early. Such an access is now recorded and settled once the enclosing top-level binding is fully inferred.

The part worth review is generalization. A variable a pending obligation depends on stays monomorphic until settled. Block-level lets do generalize, so without that guard the deferred variable would be generalized, each use would take its own copy, and the later resolution would reach none of them — it would type-check and mean nothing. That failure mode is silent, which is why the guard is in generalize rather than relying on ordering.

Deferring never weakens the check. Three tests cover the ways it must still fail: the field's type has to fit every use (l + 1 where l is the string field is still an error), the field has to exist on the record that wins, and a value nothing pins anywhere is still an error.

The message for that last case now names both ways out, since it offered only the pattern form — the parameter form did not exist when it was written, and per the report it reads better:

field `letter` is ambiguous here: nothing says what this value is, and `letter` is
declared by records `Placed` and `Cell`. Name the record where the value arrives —
in a parameter (`fun (Placed { letter }) -> …`) or in a pattern
(`case Placed { letter }:`) — or give it a type some other way

Tests: 5 typecheck cases and an end-to-end one confirming the deferred access lowers to the ordinary attribute read. Full suite, clippy, fmt and the 23 doc lessons clean.