Module filter 

RenderGrid + FilterChain framework (E012 US1/US5 — Phase 4/5).

Public types: filter::RenderGrid, filter::Cell, filter::Color, filter::Filter, filter::FilterChain. Individual filter implementations are gated behind their respective filter-<name> leaves (see Cargo.toml). The filter::Filter::Nothing identity has no leaf — always available — so an empty chain or an all-Nothing chain compiles on any feature surface. RenderGrid + FilterChain (E012 US1/US5 — FR-002, FR-003, FR-004).

This module hosts the typed grid that filters operate on, the [Filter] enum enumerating the 10 supported transformations, the [FilterChain] orchestrator, and the per-filter pure-function implementations. Each individual filter is gated behind its leaf feature (filter-crop, filter-gay, filter-metal, filter-flip, filter-flop, filter-rotate, filter-border) per ADR-0006 + plan §Cargo Feature Surface; the [Filter::Nothing] identity has no leaf and is always available.

Design constraints

• Immutability (AD-002) — every filter takes an owned [RenderGrid] and returns a new owned grid. No interior mutability, no shared borrows, no in-place transforms.
• Bounded cell footprint (AD-011) — [Cell] is ~16 bytes; the grid memory is O(w·h) and a chain of n filters costs O(n·w·h) (HINT-006 + FR-030).
• No upstream-source consultation — implementations derived from the toilet(1) manpage and observed outputs; recorded under docs/tlf-derivation.md.

Structs

Cell

A single rendered cell: a character plus optional color attributes.

FilterChain

Ordered list of Filters applied left-to-right in FilterChain::apply (FR-004).

RenderGrid

A 2D grid of Cells with explicit width × height dimensions.

Enums

Color

Color carried by a Cell — bounded footprint per AD-011.

Filter

The 10 supported toilet-compatible filters (FR-003).

NamedColor

One of the 16 ANSI named colors.