BOUNDARY: write a regex as data, get a Pattern the existing subsystem
already knows how to run.

  (api/->expr :java [:cat :start [:+ :digit] :end])
  ;; => {:ok "^\\d+$"}

  (api/register! #:pattern{:id :audit/probe} [:cat "ready" [:? "?"]])
  (pattern/match? :audit/probe "ready?")

A Construct projects INTO the existing `Pattern` value object rather than
replacing it: `:pattern/expr` is filled from the construct, `:pattern/construct`
records what it was built from, and everything downstream — the registry's
validation, `IPatternEngine`, `match?`, `scan` — is untouched. A pattern
written as a literal string keeps working exactly as before.

`register!` goes through `pattern.registry/register-pattern!` so a construct
that projects to an invalid Pattern is refused at the same gate as every
other pattern.

PROMOTE layer: lift authored regal syntax into a validated Construct, project
it back, and say what a Construct requires of its target.

Everything here is pure and total — a malformed form yields a Result
describing why, never an exception and never a silent repair.

  (normalize [:cat "a" [:alt "b" "c"]])  => {:ok {…}}
  (->regal   normalized)                    => [:cat "a" [:alt "b" "c"]]
  (required-capabilities normalized)        => #{}

`required-capabilities` is what makes dialect support a set-difference
decidable BEFORE emission rather than a `try`/`catch` around a compile.

The DIP swap point: a regex target language as data, plus the one gate every
emitter passes through.

## The substitutability contract

For a Construct a dialect cannot express, `emit` returns
`(r/err :construct/unsupported …)`. It NEVER emits an approximation and never
drops a node. A `:lookahead` quietly discarded for RE2 produces a DIFFERENT
LANGUAGE that still compiles and still returns hits — the worst outcome
available, because nothing reports it.

## Why the gate is here and not in the emitter

`emit` is a plain function that checks capabilities and THEN calls
`-render`. An implementation supplies rendering only; it cannot skip,
weaken or forget the check, because it never sees it. One definition, N
implementations.

The dialect profile is an ARGUMENT throughout — resolved from the registry
at call time, never captured at wiring time.

POSIX ERE — the dialect `grep -E` speaks, rendered without regal.

## Why this emitter is hand-written

regal has no ERE flavor, and its `:java`/`:ecma` output is not ERE: a nested
alternation comes out as `(?:a|b)`, which POSIX ERE has no syntax for. So
this is the one dialect that renders from the Construct directly. It is
small because ERE is small — literals, classes, alternation, greedy
quantifiers, anchors, and one grouping syntax.

## Why the gate matters more here than anywhere else

`rg` rejects what it cannot do: an unsupported pattern exits 2 with a
message. GNU grep 3.11 does not. Measured:

  grep -E '\d+'      matches a literal `d`  — no warning, exit 0
  grep -E '(?:a|b)c'  matches `abc`          — a warning, exit 0
  grep -E 'ab*?'      greedy                 — silently not lazy
  grep -E '(?=x)'     never matches          — a warning, exit 1

Every one of those is a DIFFERENT LANGUAGE that still runs and still returns
hits. For rg the capability gate converts a stderr parse into data; for grep
it converts a silently wrong answer into a named refusal, which is the
stronger reason the gate exists at all.

## What `:posix-ere` therefore provides: nothing

Its capability set is empty. `\d` must be written `[:class [\0 \9]]`,
which renders `[0-9]`; a tab must be a literal tab inside a `:literal`, not
`:tab`; a `:capture` is refused because ERE's only group is a capturing one,
so any structural group this emitter needs would shift the very index
`:match/groups` promises.

Everything the gate lets through is rendered exactly, or not at all.

COLLECT layer: an anti-corruption layer over lambdaisland/regal, and the
three dialect profiles it can render.

Regal is the emitter we buy rather than build — it owns escaping, precedence
and grouping, and it is the only code here that knows what a regex looks
like. Its vocabulary does not cross into the domain: it throws, we return a
Result; it names flavors, we name dialects.

## What regal does not do, and this layer must

Measured on 0.1.175: regal does NOT gate on flavor. `[:atomic "ab"]` renders
as `(?>ab)` under `:ecma`, where JavaScript has no atomic groups, and an
unknown flavor is accepted silently. The capability gate in
`construct.dialect` is therefore load-bearing, not ceremony.

## Dialects shipped

  :java  every capability            flavor :java9
  :ecma  no atomic groups            flavor :ecma
  :re2   no lookaround, no atomic    flavor :ecma  (rg, sd, RE2, Rust regex)

`:posix-ere` is deliberately absent: ERE has no non-capturing group, so
regal's `(?:…)` output is not ERE even after a capability gate. It needs a
real second emitter, which this design declines to build.

Malli value objects for regex CONSTRUCTS — a regex expression reified as data.

## Two forms, one language

A construct is AUTHORED in lambdaisland/regal's vector syntax — `[:cat "a"
[:alt "b" "c"]]` — and NORMALIZED into the recursive map value object this
namespace defines. The authored form is what callers write; the normalized
form is what carries contracts, generators and tests. `construct.core`
promotes one into the other and projects back.

The split is forced: malli refuses a `:ref` reachable inside a sequence
schema (`::m/potentially-recursive-seqex`), and regal's head-plus-variadic
vector IS a sequence schema.

## Capabilities

A `Capability` names a regex feature whose availability differs across target
languages. A `Dialect` declares the set it PROVIDES; a construct's required
set is computed from its shape. Support is a set-difference, decidable before
emission — never a `try`/`catch` around a compile.

These schemas are the single source: they drive the `m/=>` contracts and the
synthesized property/mutation tests.