refactor: modernize ext::optional shim to std::optional (C++17), remove QL_USE_STD_OPTIONAL

[tobydrinkall]

Summary

Replaces all ext::optional/ext::nullopt usages with std::optional/std::nullopt directly, removes the dead QL_USE_STD_OPTIONAL build flag (it was already defaulted ON), and reduces ql/optional.hpp to a thin deprecated forwarding header (#include <optional>).

Since QL_USE_STD_OPTIONAL was ON by default, ext::optional already resolved to std::optional — so this is a type-alias-only change with zero semantic impact. The Boost path (boost::optional) is eliminated; it was already guarded by a #error on Boost ≥ 1.91 due to silent behavior changes.

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Dependency Blast Radius

Metric	Count
ext::optional occurrences migrated	261
ext::nullopt occurrences migrated	130
#include <ql/optional.hpp> → #include <optional>	53
Total files touched	81
QL_USE_STD_OPTIONAL references removed	18 (9 files)

Migration Order (commits follow this bottom-up order, each builds green)

1. Core (settings.hpp, event.hpp, cashflow.hpp + .cpp) — price-control APIs
2. cashflows/ (8 files)
3. instruments/ (21 files)
4. pricingengines/ (16 files)
5. termstructures/ (11 files)
6. experimental/ + time/ (10 files)
7. test-suite/ (9 files)
8. Swap all #include directives
9. Remove QL_USE_STD_OPTIONAL from build system + reduce shim

Flag Teardown

Removed from: CMakeLists.txt, CMakePresets.json (both *-nonstandard-options presets), configure.ac, ql/userconfig.hpp, ql/config.hpp.cfg, .ci/userconfig{2019,2022,2026}.alt.hpp, Docs/pages/config.docs.

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Verification Results

Test-suite: 100% passed, 0 failed (182 suites / 1,321 cases, 306.85s).

Numeric equivalence (master vs branch, 20 examples):

Example	Result
EquityOption, Bonds, BermudanSwaption, FRA, MulticurveBootstrapping, CDS	IDENTICAL
AsianOption, BasketLosses, CVAIRS, CallableBonds, ConvertibleBonds	IDENTICAL
FittedBondCurve, Gaussian1dModels, LatentModel, MarketModels	IDENTICAL
MultidimIntegral, Replication, Repo	IDENTICAL
DiscreteHedging, GlobalOptimizer	inherently non-deterministic*

18/18 deterministic examples: byte-identical (2,147 total lines compared).

*DiscreteHedging (MC) and GlobalOptimizer (differential evolution) are inherently non-deterministic: two sequential runs of the same master binary also produce different output, confirming stochastic RNG-seeded processes unrelated to the migration.

Zero leftovers:

grep 'ext::optional\|ext::nullopt' ql/ test-suite/  → 0
grep 'QL_USE_STD_OPTIONAL' (source)                 → 0
grep 'boost::optional\|boost::none' (source)        → 0

Link to Devin session: https://app.devin.ai/sessions/a007a5cdfb1842729b8875e744ed3b8b
Requested by: @tobydrinkall

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✅ Devin Review: No Issues Found

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