Fix 64-bit integer truncation and REAL precision loss on fetch (#17)

[jkalias]

Summary

Fixes the silent data-corruption defects described in #17 (which consolidates #15 and #16).

FetchRecordsQuery::GetColumnValue (src/queries.cc) stringifies every fetched column into a std::wstring before it's parsed back into the typed member. Two of these conversions were lossy on read:

• SQLITE_INTEGER was read with the 32-bit sqlite3_column_int, silently truncating/wrapping any stored int64_t above INT32_MAX (large explicit ids, epoch-millis timestamps, big quantities, and AUTOINCREMENT ids once they grow).
• SQLITE_FLOAT was formatted with std::to_wstring(double), which uses %f with a fixed 6 decimal places — e.g. 0.123456789 round-tripped back as 0.123457, and large-magnitude doubles were mangled.

Writes were already correct (sqlite3_bind_int64 / sqlite3_bind_double store the true value); the loss was purely on the read path.

Fix

This is approach A from the issue — minimal and text-path-preserving:

• Integers: read with sqlite3_column_int64 instead of sqlite3_column_int.
• Doubles: format with %.17g (full round-trip precision) via snprintf, instead of std::to_wstring.

TEXT, BOOL, and DATETIME handling is untouched. The parse side (StringUtilities::ToInt → std::stoll, StringUtilities::ToDouble → std::stod) already round-trips these representations exactly, so no changes were needed there.

Note on scope: this fixes both data-corruption bugs and satisfies the two round-trip acceptance criteria in #17, but it does not remove the intermediate std::wstring round-trip — so it doesn't meet #17's "hydration reads typed columns directly" bullet (the direct-hydration/streaming redesign). I'd suggest keeping #17 open (or retitling it) to track that as a separate performance follow-up, since it's a larger, more invasive change touching FetchQueryResults and its consumers.

Out of scope, left untouched per the issue boundaries: #19 (ToInt/ToDouble returning 0 on parse failure), #20 (NULL vs empty-string conflation), #14 (identifier quoting), relationships/nullable fields/predicates.

Test plan

• Added DatabaseTest.FetchPreservesInt64ValuesBeyondInt32Range — saves Company records with age values above INT32_MAX (up to near INT64_MAX), fetches them back, asserts exact equality.
• Added DatabaseTest.FetchPreservesHighPrecisionDoubleValues — saves Company records with a high-precision fractional salary and a large-magnitude salary, fetches them back, asserts exact equality via EXPECT_DOUBLE_EQ.
• Confirmed both new tests fail against the pre-fix code (observed truncated/wrapped integers and 0.123457 instead of the full-precision double).
• Confirmed both new tests pass after the fix.
• Ran the full suite (cmake --build build && ./build/tests/unit_tests) — all 60 tests pass.

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