fix: add missing build() method and align docs with actual API (TryFrom<&str> / FromStr)

[ZialeHub]

Summary

The README and multiple inline doc comments reference a build() constructor method on Date, Time, and DateTime:

// From the README:
let mut time = Time::build("T23:17:12Z.000")?;
let mut datetime = DateTime::build("2024-10-31 06:32:28")?;

// From date.rs doc comments:
let date = Date::build("2023-10-09")?;

// From time.rs doc comments:
let time = Time::build("12:21:46")?;

// From datetime.rs doc comments:
let datetime = DateTime::build("2023-05-17 09:05:12")?;

None of these compile. No build method exists on any of the three types. The nearest equivalent is TryFrom<&str>:

let date = Date::try_from("2023-10-09")?;

This is a documentation correctness bug that directly misleads new users, causes the README code examples to fail if copied, and degrades trust in the library.

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Root Cause

The build naming was used in early design sketches or a previous API version and was referenced in documentation without being implemented. The Span trait's new(y, m, d) constructor is for component-based construction; string-based construction was left only as TryFrom<&str> with no named method alias.

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Proposed Fix

Option A — Add build() as a named constructor (recommended)

Add a build inherent method to each type that delegates to TryFrom<&str>:

impl Date {
    /// Parse a `Date` from a string using the active `BASE_DATE_FORMAT`.
    ///
    /// # Example
    /// ```rust
    /// let date = Date::build("2024-10-31")?;
    /// assert_eq!(date.to_string(), "2024-10-31");
    /// ```
    pub fn build(s: &str) -> Result<Self, SpanError> {
        Self::try_from(s)
    }
}

Identical additions for Time::build(s: &str) and DateTime::build(s: &str).

This approach:

• Makes all README examples compile as written
• Adds zero new logic (pure delegation)
• Is consistent with the builder-pattern theme of the crate (SpanBuilder::builder())
• Pairs naturally with the FromStr implementation proposed in issue feat: implement std::str::FromStr for Date, Time and DateTime #54

Option B — Update all docs to use TryFrom

Replace every Type::build("...") reference in the README and doc comments with Type::try_from("...") or "...".parse::<Type>(). Lower effort, but makes the README less readable and requires updating every example.

Option A is strongly preferred — the method name build is already established in the library's mental model and documentation.

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Affected Locations

File	Reference
README.md	Lines referencing Time::build, DateTime::build
date.rs	Doc comment on clear_unit
time.rs	Doc comment on clear_unit
datetime.rs	Doc comment on clear_unit

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Acceptance Criteria

• Date::build(s: &str) -> Result<Self, SpanError> added
• Time::build(s: &str) -> Result<Self, SpanError> added
• DateTime::build(s: &str) -> Result<Self, SpanError> added
• All three methods delegate to TryFrom<&str> and use BASE_*_FORMAT
• All README code examples compile as written (cargo test --doc passes)
• All inline doc comment examples compile (cargo test --doc passes)
• build methods are exported from prelude via the type re-exports (inherent methods, so this is automatic)
• Span trait updated or doc-commented to reference build as the string-parsing entry point