refactor!: DynDisplay API to mirror std::fmt::Display

[qjerome]

Summary
Refactor the DynDisplay trait API to mirror std::fmt::Display, introducing a Formatter type that implements std::fmt::Write for natural use of write! inside formatting implementations.

Motivation
The previous API returned Result<String, Error> from dyn_fmt, requiring manual alignment and intermediate allocations. The new design follows Rust's standard formatting conventions, enabling zero-allocation streaming to the output sink and leveraging the familiar write! macro.

Changes

• Add Formatter<'a> struct bundling FormatSpec and output sink, implementing std::fmt::Write
• Add pub type Result = std::result::Result<(), Error> for trait consistency
• Change DynDisplay::dyn_fmt signature from fn(&self, &FormatSpec) -> Result<String, Error> to fn(&self, &mut Formatter<'_>) -> Result
• Replace FormatSpec::fill_and_align with write_aligned that writes directly to sink (removes per-argument String allocation)
• Rename pipeline driver from Formatter<'s> to Writer<'s> to avoid naming conflict
• Add Error::Write variant for fmt write failures
• Add Error::NamedArgument variant; named/positional arguments ({0}, {name}) now return Err instead of silently formatting positionally
• Update all standard type implementations in src/imp.rs to use new API
• Update documentation and examples throughout

Breaking Changes

• DynDisplay::dyn_fmt signature change requires all implementers to migrate
• Formatter struct renamed to Writer in the public API
• fill_and_align removed; alignment now handled automatically via Formatter::finish

Testing
Existing test suite passes with updated implementations. All dformat! usage remains unchanged.

Notes
This aligns the library's mental model with std::fmt, making custom implementations more intuitive for Rust developers.