feat(engine): add get_input_source and get_input_color helpers for Pine input mapping #23

Context

Cross-repo blocker for pineforge-codegen #9. Two Pine v6 input variants currently route to wrong/scalar engine getters because no proper helper exists.

Missing helpers

1. `get_input_source(name, const Series& default_series) -> const Series&`

Problem: Codegen routes `input.source` to `get_input_double` (scalar). Generated C++ emits e.g. `double src = 0.0; src = get_input_double("Source", current_bar_.close);` — `current_bar_.close` is captured ONCE at the call site, so any downstream `ta.sma(src, 14)` sees the same bar-0 close every bar = frozen scalar instead of a live series.

Required behavior: Engine receives a string key ("open" / "high" / "low" / "close" / "hl2" / etc.) at runtime, resolves to the matching native OHLCV Series reference, returns that reference. If no input override, returns the default series passed in (typically `_s_close`).

Analyzer contract (already enforced in pineforge-codegen): `input.source` defval MUST be a native chart series (open/high/low/close/hl2/hlc3/ohlc4/...). See codegen `_validate_plain_input_source`.

2. `get_input_color(name, int32_t default_rgba) -> int32_t`

Problem: Codegen routes `input.color` to `get_input_int`. Acceptable for now because the analyzer guard in codegen PR #13 restricts defval to `color.` / `color.new(...)` / `color.rgb(...)` — all of which already lower to packed rgba int32 via `pine_color::new_color`. The semantic gap: the runtime key→value lookup is via string parsing (`std::stoi`) which works for integer literal but is semantically wrong for a color input.

Required behavior: Mirror `get_input_int` but document the value contract as packed rgba (R<<24 | G<<16 | B<<8 | A). Could be the same impl as `get_input_int` with a renamed entry point so codegen can route per-type.

Lower priority than get_input_source — current behavior produces correct values for the constrained defval set.

Out of scope

• `input.time` already fixed via `get_input_int64` (PR #22, merged 2026-05-28).

When this lands

After both helpers are in the engine, pineforge-codegen `_input_type_to_getter` (`pineforge_codegen/codegen/input.py:102-122`) routes:

• `func_name == "source"` → `get_input_source`
• `func_name == "color"` → `get_input_color`

Then codegen #9 closes fully.