feat: Add consolidated snippets.

[kinyoklion]

First slice of snippets/: a self-contained Go module that owns the canonical source for LaunchDarkly SDK code samples and renders them into downstream consumers. Scope: python-server-sdk "Getting Started" flow, end-to-end.

In scope

• Snippet file format (YAML frontmatter + one fenced code block).
• Generator CLI (render, verify, validate).
• ld-application adapter — rewrites the body of marked <Snippet> elements in TSX.
• Docker validator that runs the snippet verbatim against a real LD environment.

Out of scope (later slices)

• All SDKs other than python-server-sdk.
• ld-docs adapter, GitHub Actions, signed-binary release, sdk-meta capability integration, region/version conditionals.

Snippet file

---
id: python-server-sdk/getting-started/install
sdk: python-server-sdk
kind: install
lang: shell
inputs:
  version: { type: string, runtime-default: "" }
---

```shell
echo "launchdarkly-server-sdk{{ if version }}=={{ version }}{{ end }}" >> requirements.txt && pip3 install -r requirements.txt

Templating: `{{ name }}` substitutes; `{{ if name }}…{{ end }}` is a non-empty conditional. The `ld-application` adapter renders these as JS template-literal expressions: `${name}` and `${name ? \`…\` : ''}`.

## Render markers

Markers sit in the host file's own comment syntax. The generator replaces only the children of the JSX element that follows. Marker hash + version are filled in by `snippets render`.

```tsx
{/* SDK_SNIPPET:RENDER:python-server-sdk/getting-started/install hash=8e6373549d36 version=0.1.0 */}
<Snippet lang="shell" withCopyButton>{`echo "launchdarkly-server-sdk${version ? `==${version}` : ''}" >> requirements.txt && pip3 install -r requirements.txt`}</Snippet>

verify recomputes the hash and fails on hand-edits inside marked regions.

CLI

# Rewrite all marked regions in a consumer checkout
snippets render --target=ld-application --out=/path/to/app

# Fail if anything drifted (intended for consumer CI)
snippets verify --target=ld-application --out=/path/to/app

# Run the snippet in Docker against a real LD environment
export LAUNCHDARKLY_SDK_KEY=…  
export LAUNCHDARKLY_FLAG_KEY=…
snippets validate --sdk=python-server-sdk

Test plan

• cd snippets && go build ./... && go test ./... clean.
• snippets render --target=ld-application --out=<app> is idempotent on a checkout that already contains the markers.
• snippets verify rejects a hand-edit inside a marked region.
• snippets validate --sdk=python-server-sdk against a real LD test env prints *** The <flag> feature flag evaluates to <value> and exits 0.