Move SuggestionCoordinator's snapshot-to-situation mapping into a covered type

## What happens

`Sources/Uttrflow/Suggestion/SuggestionCoordinator.swift` (770 lines) is excluded as "wiring only". Three private functions in it are pure mappings from a `FocusedFieldSnapshot` to what the model and the capture store see:

- `situation(...)` (line 556): the `GenerationSituation` handed to the model;
- `reading(of:)` (line 752): the `FieldReading` used for capture refusals and corpus grouping;
- `context(of:at:)` (line 762): the `PredictionContext`.

No test reaches them, and the exclusion reason says every rule is `SuggestionSession`, which isn't true of these.

Measured on origin/main `26d7bc1` with `swift test --enable-code-coverage` (5,384 tests, all passing) and `xcrun llvm-cov export` over the test binary. The regions named below have an execution count of 0: no test runs them, so a change that breaks them passes `make verify`.

## Why it matters

`reading(of:)` decides the bundle identifier, role and secure-field flag that `CaptureGate` refuses on (#644, #668). A mistake there, such as dropping the subrole, makes a password field look like an ordinary one to the refusal rules that are tested. `situation` decides which screen text reaches the model.

## How to reproduce

Read `SuggestionCoordinator.swift:556-577` and `:752-770`, and run `grep -rn "FieldReading(" Tests/UttrflowTests`.

## Acceptance criteria

- The three mappings become internal static functions on a type in a covered module (for example beside `FocusedFieldSnapshot` in `UttrflowContext`, or in `UttrflowPredictCapture`), taking a snapshot and returning a value.
- Tests assert that a secure snapshot produces a secure `FieldReading`, that the bundle identifier is passed through with its case unchanged, and that the situation contains the preceding text and screen lines the snapshot holds.
- The exclusion reason for `SuggestionCoordinator.swift` stays accurate.
- Each new test fails when the branch it covers is broken. Check this by hand before opening the PR: temporarily change the branch (flip the condition or return the other value), see the test fail, then restore it.

Read Sources/Uttrflow/Suggestion/SuggestionCoordinator.swift:556-577 and :752-770, then run grep -rn "FieldReading(" Tests/UttrflowTests. Move the three mappings to a covered type and add tests under Tests/UttrflowTests for secure fields, unchanged bundle identifiers, and snapshot text; verify each test fails when its mapped branch is temporarily broken.

Written by the indexing model from the issue text.