Add build-wide default optimization level (size) option

[stephenberry]

Summary

Lets embedded users make size the default optimization level for the whole build, instead of remembering to pass glz::opts_size{} at every call site. A single forgotten default-options call (e.g. glz::write(obj, buf), or a transitive glz::format_error) can no longer silently relink the 40 KB integer digit_quads table, the ~16 KB float pow-10 tables, or per-struct hash tables.

This is a follow-up to the discussion in #2614 and folds in the error-formatting fix from #2620. It reuses the existing value-based optimization_level machinery rather than adding a parallel #ifdef integer-only path.

Mechanism

• optimization_level.hpp — adds an exported glz::default_optimization_level constant, fed by GLZ_DEFAULT_OPTIMIZATION_SIZE (boolean knob) or GLZ_DEFAULT_OPTIMIZATION_LEVEL (explicit level, advanced).
• opts.hpp — routes the check_optimization_level and check_linear_search fallbacks through that constant, so integers, floats, and hash-table elimination flip together. Explicit per-call options still win: an optimization_level::normal keeps the fast tables even in a size-default build, so the only path back to the large tables is explicit and greppable. The default (normal) build is byte-for-byte unchanged.
• CMakeLists.txt — glaze_DEFAULT_OPTIMIZATION_SIZE option injects the macro as an INTERFACE compile definition (build-wide, ODR-consistent).

Internal leak fixes (so size users actually get the guarantee)

• validate.hpp — glz::format_error now formats index/line/column with opts_size{} (these are size_t only; error formatting is never a throughput path). This is the specific footgun raised in Add cmake option to optimize for size #2614 and the change from Use size optimization for error formatting #2620.
• jsonb/write.hpp — BEVE float writes preserve the caller's optimization level instead of hardcoding normal opts, so size-optimized BEVE writes drop the float pow-10 tables.

C++20 modules

default_optimization_level is a value consumed via if constexpr (is_size_optimized(Opts)), never a preprocessor branch inside serialization code. When Glaze gains a module interface, the default is baked once at module-build time and consumed everywhere by name, while per-call glz::opts selection (the only behavior driver) composes with import glaze; with zero macros.

Verification

tests/size_optimization_test:

• Compiles the wiring checks in both modes (static_assert): a bare opts{} inherits the build default for ints/floats/linear-search; opts_size{} is always size; an explicit normal still wins.
• Asserts via nm that a size-default binary does not link the 40 KB digit_quads table, so a future hardcoded-normal regression fails CI. (Verified locally: size build has 0 such symbols, a normal control has 1.)

Built and ran locally (AppleClang, Debug): size_optimization_test, size_optimization_size_default, size_optimization_symbols, plus json_test, beve_test, lazy_beve_test, jsonb_test, csv_test, float_write_test all pass.

Closes the design question in #2614; supersedes #2620.