Docs accuracy: fix fabricated and stale claims across rustdoc and READMEs

[RAprogramm]

Three audits (README.template.md, README.ru.md/README.ko.md, rustdoc prose) found the following. Fix docs to match actual behavior.

Fabrications

• README.template.md:581 — AppError::with_retry_after_duration(...) doesn't exist (only with_retry_after_secs on AppError; with_retry_after_duration is on ErrorResponse). Snippet doesn't compile. Also stale in src/lib.rs doc.
• README.template.md:504 — request_ref::<TelemetrySnapshot> through AppError converted via #[app_error] panics: the generated From impl discards the domain error (see #[app_error] From conversion drops source and telemetry #452-followup issue).
• README.template.md:593-630 — DisplayMode advertised as controlling Display output; Display for Error never consults it (see followup issue).
• src/convert/redis.rs:24 — rust,ignore example asserts AppErrorKind::Cache for an IO error; actual mapping promotes to DependencyUnavailable.
• src/app_error/core/introspection.rs:277 — downcast_mut documented as functional; body is unconditionally None (see followup issue).
• src/lib.rs:70 + README.template.md:89,138 — feature telegram-webapp-sdk doesn't exist; actual feature is init-data (crate init-data-rs).

Stale

• src/lib.rs:49 — MSRV 1.90 → 1.96.
• Five convert modules (serde_json, teloxide, init_data, redis, multipart) claim 'original error string preserved in message'; since Context refactor the message is None and text goes to metadata/source.
• src/convert/actix.rs — claims empty-body mode without serde_json; no such path (actix feature hard-wires serde_json).
• src/convert/init_data.rs:23 — apostrophe fences instead of backticks; example never renders/compiles.
• src/app_error/core/backtrace.rs:98,125 — doc examples cfg(test)-guarded, never compiled; private-path imports.
• src/response.rs:15 — gRPC wire fields are grpc.name/grpc.value, not grpc.code.
• README.template.md:705 — 'all examples include comprehensive tests': only axum-rest-api has tests.
• README.template.md:91 — turnkey 'tracing instrumentation': no tracing code in src/turnkey/.

Imprecise (see audit for full list, ~15 items)

• 'everything off by default' (default = ["std"]); request_ref snippets need nightly (undocumented); sqlx feature maps sqlx_core::Error; axum enables dep:serde_json but not the serde_json feature; reqwest impl-doc contradicts module-doc on status mapping; downcast_ref downcasts the source, not the error; AppCode::try_new underscore rules; 'redacts when marked private' wording on axum/actix impls; etc.

README.ru.md / README.ko.md (~4 minor versions behind)

• version 0.24.19 → 0.28.0 (2 places each); telegram-webapp-sdk → init-data in install snippet; add colored to snippet; MSRV badge/footer 1.90 → 1.96; license 'MIT OR Apache-2.0' → MIT (badge + footer); add missing Examples section + TOC entry; add REUSE badge.