codegen: a calc_on_order_fills restore no longer rolls back requested-evaluation state - #260

…ialization, request.currency_rate, helper-local var color, one-line field assignment) (#261)

## What

This branch carries five codegen fixes planned for 1.8.0. It is five merge commits on `main`, one per pull request, in this order, followed by one CHANGELOG commit.

| Merged pull request | Fix |
|---|---|
| #260 | A `calc_on_order_fills` recalculation after a fill no longer rolls back the state of a deferred `request.security` evaluation (a silent wrong value, for example after a late margin-call liquidation). |
| #259 | A persistent (`var`) UDT, array, map or matrix whose initializer reads an earlier computed value is initialized once, in statement order (Fixes #226). A `varip` declaration with the same dependency still reads the default; that gap is tracked in #258. |
| #254 | `request.currency_rate` raises its error only where the call is evaluated, not where it merely appears in a branch that never runs (Fixes #228). |
| #255 | `var color` state inside a `request.security` helper, explicit or inferred, with persistence, transparency and `na` (Part of #229). The refusal for any other unsupported helper-local `var` type has the new code `PF-E3093`. |
| #253 | A UDT field assignment as the body of a one-line function or method (Part of #230). |

The CHANGELOG commit puts all five entries in the planned 1.8.0 section, updates its summary and compatibility notes, and removes the separate planned-patch section one of the merged branches added.

## Scope and limits

- The five merge commits resolved no code conflict; the only textual conflicts were in `CHANGELOG.md`, replaced once by the last commit. The name-status against `main` is the union of the five pull requests' own files (28 files).
- Over the 555 corpus and fixture scripts the emitted C++ differs from `main` in 72 scripts, all from the #260 change and only inside `restore_script_state()`; the other 483 are byte-identical. The other four fixes change no script of the 555; their tests cover their shapes.
- The user-facing text of two diagnostics (`PF-E1088`, `PF-W1088`) is reworded and one code is added (`PF-E3093`); the catalog schema is unchanged.
- The second half of #229 (the history of a derived helper argument) and the `varip` gap (#258) are not in this branch; they follow as separate pull requests.

## Tests

- Full suite on the head of this branch with the paired engine: 10725 passed, 221 skipped (private fixtures and empty parameter sets, the same 221 as `main`), 0 failed.
- A parity sweep over the whole population (7989 graded probes): PASS, no regression and no improvement (hard gate: 1009 evaluated, no moves; coverage complete).
- Every pull request above keeps the tests it was reviewed with; this branch adds no test of its own.

Fixes #226, Fixes #228. Part of #229, Part of #230.

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… bound to a derived expression (#262)

## What

`request.security` now accepts a helper that reads the history of a parameter bound to a derived expression, as TradingView does. The reporter's script from #229 translates:

```pine
f(x) => x[2] - x
a = ta.sma(close, 3)
b = f(a)
c = request.security(syminfo.tickerid, '60', b)
```

Before, the transpiler refused it ("helper-parameter history currently requires a direct OHLC/time bar-series binding"). The helper's `x[n]` read the chart's bars; it now reads the argument's own values on the requested context's bars.

This is the second half of #229. The first half (`var color` helper state) is in #261.

## How it works

- Each call site that needs the history keeps its own ring of argument values, sampled on the requested context's bars, so `x[n]` returns the value the argument had `n` evaluations of that call ago.
- Admission is by the position of the call. Direct OHLC and time bindings, builtin forms and method envelopes keep their existing lowering.
- The ring is requested-evaluation state. The restore after a `calc_on_order_fills` fill (#260, in #261) leaves it alone, like the rest of the requested progression state.
- The history of a global that is fully overwritten before the request statement replays its last unconditional write.

## Refusals

Shapes that cannot be sampled are refused with located errors, `PF-E3094` through `PF-E3101`: a derived history offset that is not a non-negative constant integer, a ring capacity that is not representable, a call that is not unconditional, a helper that depends on other helpers' state, history composed with helper-result history, a mutable global that cannot be replayed, and a parameter whose type or width has no lossless history. The code numbers follow `PF-E3093` from #261; messages are unchanged by the numbering.

## Scope and limits

- Over the 555 corpus and fixture scripts the emitted C++ is identical to main: none of them contains a derived helper-argument history, so the change reaches only scripts of the shape above (the tests cover them).
- One diagnostic code range is added (`PF-E3094` to `PF-E3101`); the catalog schema is unchanged. The CHANGELOG entry is in the 1.8.0 section.

## Tests

- Full suite with the paired engine: 10837 passed, 221 skipped (private fixtures and empty parameter sets, the same 221 as main), 0 failed. The branch adds the spec test of the reporter's script (red on main), a compiled-and-run witness of the sampled values, a refusal test with one row per shape, and a witness that the ring survives a late fill recalculation.

Fixes #229

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