fix(codegen): request.security TA expression read at a history offset stays in HTF context

[luisleo526]

Problem

request.security(tf, ta.<fn>(...)[k], lookahead=...) — a security expression that is a TA call read at a history offset — was mis-transpiled. In security.py::_build_security_expr the Subscript branch only handled Subscript(Identifier) (OHLC fields / helper bindings). A Subscript(FuncCall-TA) fell through to the generic chart-context path, which re-lowered the inner TA to the chart member and gated a _hist_call buffer on is_first_tick_ (chart tick) instead of is_complete (HTF-bar boundary). Without a magnifier is_first_tick_ is true every bar → it produced the chart-timeframe TA, and the correctly-computed security value _secval_* was discarded.

Fix

Mirror the OHLC history-offset machinery from #30 for TA call-sites:

• declare a per-(sec, site) Series<double> {member}_hist,
• push the already-committed _secval_* gated on is_complete (so the indicator is not double-stepped),
• read hist[k-1] for offset k≥1; offset 0 reuses the bare _secval_*.

Handles scalar, tuple (mixed OHLC+TA), and lower_tf security exprs; cleared in all clear_security branches. A non-literal TA history index errors loudly.

Regression-safe: when the security tf == chart tf the engine sets is_complete every bar, so the history advances every bar (identical to chart behaviour); OHLC close[1] stays on the untouched Identifier branch. Lookahead policy unchanged.

Validation

	result
officialjackofalltrades-caldera-meridian (ta.ema(close,55)[1] @ "240")	78.5% → 97.5% match vs TV
3commas-triple-rsi-dca (ta.rsi(...)[1] in security)	no-trades → 29% (recovered)
quant-synthesis / waranyutrkm	unchanged (waranyutrkm's ta.atr(...)[1] now correctly HTF-gated)
full gate PINEFORGE_ENGINE_INCLUDE=... pytest	1384 passed, 1 skipped, 0 failed (incl. 254-strategy corpus compile sweep)
broad 100-strategy sweep	ok 88→89, transpile-errors identical set, zero regressions (byte-diff isolated: exactly 3 strategies changed, all ≥)

New test_request_security_ta_history_offset_uses_htf_gating asserts HTF gating (is_complete + security member) and the absence of the chart-context is_first_tick_/_hist_call form; fails before the fix.

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