beme public API: read and print beme syntax.

Three tracks:
  text-to-form:  beme->forms, forms->beme (all platforms)
  form-to-text:  forms->clj (all platforms), clj->forms (JVM only)
  text-to-text:  beme->clj (all platforms), clj->beme (JVM only)

Pipeline:
  beme.alpha.pipeline/run — full ctx->ctx pipeline with intermediate state

Pretty-printer: Clojure forms → idiomatic multi-line beme text.
Width-aware — uses begin/end for forms that don't fit on one line.

beme printer: Clojure forms → beme text.

Consistent error infrastructure for the beme reader/tokenizer.
All error throw sites should use `beme-error` to ensure uniform
location tracking and message formatting.

Shared form-level predicates and constructors.
Cross-stage contracts that both the parser and printer depend on.

beme reader: recursive-descent parser.
Transforms beme tokens into Clojure forms.

Value resolution: converts raw token text to Clojure values.
Centralizes all host reader delegation (read-string calls) that were
previously scattered across the parser.

Explicit pipeline composition: source → scan → group → parse → resolve → forms.
Each stage is a ctx -> ctx function operating on a shared context map.

Unified CLI. Functions accept a map — works with both clj -T and bb.

beme REPL: read beme, eval as Clojure, print result.

Run .beme files: read, eval, return last result.

Token grouping stage: collapses opaque regions (reader conditionals,
namespaced maps, syntax-quote with brackets) from flat token sequences
into single composite tokens.

This replaces the tokenizer's read-balanced-raw by operating on
already-tokenized input where bracket matching is trivial — strings,
chars, and comments are already individual tokens.

Scanner-level source-position utilities.
Defines the character-level line/col model used by the tokenizer and grouper.
Only \n advances the line counter — \r is a regular character that occupies
a column. This matches sadvance! in the tokenizer.

Note: this is the *scanner* line model, not a universal line definition.
The error display module (beme.alpha.errors/source-context) uses
str/split-lines which has different line-ending semantics (splits on
both \n and \r\n). The two models agree for LF sources but diverge
for CRLF. See format-error for how the bridge is handled.

beme tokenizer: character scanning and token production.
Transforms beme source text into a flat vector of typed tokens.