Export number and date attributes with valid XSD lexical forms

[Floating-Y]

Why

Valid normalized attributes can produce ill-typed RDF literals: JSON numbers may use exponent notation that xsd:decimal does not accept, while year, month and timestamp values are all labeled xsd:date.

This is a follow-up to #865 and the export audit discussed in #831. It preserves the summary/class and missing-object behavior already fixed by #865.

What changes

• Expand exponent notation exactly through the existing BigDecimal type, retaining xsd:decimal without another floating-point conversion or fixed-scale rounding. Bound expansion to the exponent range produced by normalized finite numbers.
• Select gYear, gYearMonth, date or dateTime from the date value. Format calendar fields without truncating negative or extended years.
• Preserve offsets and all fractional-second digits. Complete reduced hour/minute clocks with the zero fields required by XSD, normalize supported timezone spellings, and leave timezone-free clocks timezone-free.
• Preserve value precedence, string-valued summary/class and relative values, and omission of unresolved objects.
• Replace three invalid numeric fixtures with valid numeric examples while retaining their original precedence and resolution assertions.

Examples:

Before	After
"1e-7"^^xsd:decimal	"0.0000001"^^xsd:decimal
"1e+20"^^xsd:decimal	"100000000000000000000"^^xsd:decimal
"2024"^^xsd:date	"2024"^^xsd:gYear
"2024-07"^^xsd:date	"2024-07"^^xsd:gYearMonth
"2024-07-15T14Z"^^xsd:date	"2024-07-15T14:00:00Z"^^xsd:dateTime

Changes are confined to rdf.rs and its tests. There are no dependency, storage, input-validation, ontology-datatype or unrelated export-structure changes. parse_time is used only for calendar-only values, so its timestamp truncation cannot discard an attribute's clock or fractional seconds.

XSD dateTime alone cannot encode source hour/minute precision; this patch adds no precision predicates. Unrecognized text continues through the existing fallback, including malformed clock text that today's permissive input validation may accept.

How it was checked

• Added regressions before implementation: the original implementation passed 25 RDF tests and failed the three new regression tests.
• Added a failing calendar-boundary regression during review, then corrected negative/extended-year formatting.
• All 30 final RDF tests pass. They independently assert lexical values and datatypes across Turtle and JSON-LD, ordinary fact objects, current triples, qualifiers and derived facts. Coverage includes negative numbers, zero, ordinary decimals, the smallest/largest finite f64 values, a stored integer beyond exact f64 precision, reduced clocks, timezone variants and fractional seconds beyond nanosecond precision.
• cargo fmt --all --check
• cargo clippy --locked --offline --workspace --all-targets -- -D warnings
• cargo test --locked --offline --workspace --no-fail-fast
• git diff --check

The final Windows workspace run passed with CLI temporary-directory writes permitted and NO_PROXY=localhost,127.0.0.1,::1 for the test process. Database-backed tests skipped because UTOPIA_DATABASE_URL was unset; six existing ignored tests were not run. Frontend checks were not run because no frontend files changed.

Before review

• Every commit is signed off (git commit -s).
• Format, workspace Clippy and workspace tests pass, with the environment and skipped-test limitations above.