feat(core)!: zero-allocation no_std encoding (0.2.0)

[ashaffah]

Summary

Reworks the entire crate to be zero-allocation: the default build is now
pure no_std and does not link alloc at all, so any stray heap
allocation becomes a compile error.

Key changes

• BarcodeEncoder::encode_into(input, &mut [bool]) -> Encoded — all 14
  symbologies (linear, EAN/UPC, GS1, postal, PDF417, Data Matrix, Aztec, QR)
  write their modules into a caller-provided buffer using fixed stack scratch.
• common::buffer::SliceWriter — bounds-checked slice writer
  (EncodeError::BufferTooSmall).
• Allocation-free EncodeError (&'static str / char payloads, no
  String/format!).
• common::svg — renders SVG into any core::fmt::Write sink, no heap.
• Optional alloc feature gates the owned encode() -> BarcodeOutput
  convenience and to_svg_string(); std implies alloc; image implies std.

Breaking changes (→ 0.2.0)

• encode() now requires the alloc feature.
• The trait's associated type Error is removed; all encoders use EncodeError.
• PDF417 codeword patterns changed so every codeword is a valid 17-module
  pattern (constant row width) — this alters the PDF417 visual pattern, which
  was already a "representative" (non-spec) encoding.

Verification

Configuration	Build	Tests	Clippy -D warnings	Miri (UB + leaks)
--no-default-features (no heap)	✅	127 ✅	✅	127, 0 leaks/UB ✅
--features alloc	✅	—	✅	145, 0 leaks/UB ✅
--all-features	✅	156 lib + 23 doctests ✅	✅	—

cargo fmt --all --check ✅. #![forbid(unsafe_code)] retained in all 29 source
files. CI gains build + test + clippy jobs for --no-default-features.

[Copilot]

Pull request overview

This PR refactors the barcodes crate to make encoding zero-allocation by default and fully no_std without linking alloc, introducing an encode_into() API that writes modules into a caller-provided &mut [bool] and returns an allocation-free Encoded shape descriptor. It also adds allocation-free SVG streaming via core::fmt::Write, and gates the owned encode() / to_svg_string() conveniences behind an alloc feature.

Changes:

• Replaces per-symbology encode() -> BarcodeOutput implementations with encode_into() -> Encoded, using fixed-capacity stack scratch + common::buffer::SliceWriter.
• Introduces allocation-free Encoded + EncodeError and adds streaming SVG writers in common::svg.
• Updates crate docs/README, feature flags (alloc, std implies alloc), and CI to validate --no-default-features (no heap).

Reviewed changes

Copilot reviewed 28 out of 29 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
src/twod/pdf417.rs	Converts PDF417 to encode_into() + stack scratch + slice writer output.
src/twod/datamatrix.rs	Converts Data Matrix to encode_into() + fixed scratch buffers and direct grid writes.
src/twod/aztec.rs	Converts Aztec to encode_into() with fixed-capacity grids/bits and no heap.
src/qrcode.rs	Ports BarcodeEncoder impl to encode_into() with stack scratch buffers and caller module buffer.
src/postal/rm4scc.rs	Converts RM4SCC to encode_into() with fixed input/state buffers and SliceWriter.
src/postal/imb.rs	Converts IMb to encode_into() with fixed digit buffer and SliceWriter.
src/linear/itf.rs	Converts ITF to encode_into() and eliminates padding allocation via virtual leading zero.
src/linear/code93.rs	Converts Code 93 to encode_into() with fixed value buffer and SliceWriter.
src/linear/code39.rs	Converts Code 39 to encode_into() with SliceWriter-based pattern expansion.
src/linear/code128.rs	Converts Code 128 to encode_into() with fixed symbol buffer and SliceWriter.
src/linear/codabar.rs	Converts Codabar to encode_into() with SliceWriter-based pattern expansion.
src/lib.rs	Makes crate #![no_std] by default and gates alloc/std extern crates behind features.
src/gs1/gs1_128.rs	Removes allocation in GS1-128 parsing/encoding by borrowing slices and fixed buffers.
src/gs1/databar.rs	Converts DataBar to encode_into() and replaces Vec-based expansion with SliceWriter.
src/ean_upc/upce.rs	Converts UPC-E to encode_into() with SliceWriter and alloc-free errors.
src/ean_upc/upca.rs	Converts UPC-A to encode_into() with SliceWriter and alloc-free errors.
src/ean_upc/ean8.rs	Converts EAN-8 to encode_into() with SliceWriter and alloc-free errors.
src/ean_upc/ean13.rs	Converts EAN-13 to encode_into() with SliceWriter and alloc-free errors.
src/common/types.rs	Introduces Encoded and gates owned output types behind alloc.
src/common/traits.rs	Replaces trait contract with encode_into() and adds alloc-gated default encode().
src/common/svg.rs	Adds allocation-free SVG rendering that streams to any core::fmt::Write.
src/common/output.rs	Reworks owned BarcodeOutput::to_svg_string() as an alloc wrapper over streaming SVG.
src/common/mod.rs	Exposes new buffer/svg modules and gates output behind alloc.
src/common/errors.rs	Makes EncodeError allocation-free and adds BufferTooSmall + InvalidCharacter.
src/common/buffer.rs	Adds SliceWriter for bounds-checked writing into caller-provided module buffers.
README.md	Documents zero-allocation core, new feature matrix, and updates examples/version.
Cargo.toml	Bumps to 0.2.0 and introduces alloc feature; makes std imply alloc.
Cargo.lock	Updates package version to 0.2.0.
.github/workflows/ci.yml	Adds clippy/build/test jobs for --no-default-features to enforce “no heap”.

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