Function is_chart_keyword_shape 

pub fn is_chart_keyword_shape(s: &str) -> Result<(), String>

Predicate: assert that s is a valid chart-keyword shape. The :etiquetas axis is a per-entry registry-search-tag identifier that lands in the rendered lareira-<nome> Helm chart’s Chart.yaml keywords: array via [caixa-helm]’s build_chart_yaml (folded through a std::collections::BTreeSet alongside the four substrate-fixed tags lareira / wasm / tatara-lisp / caixa-servico) and indexes the chart through Artifact Hub’s keyword-search axis + the future caixa-registry’s keyword index. The contract — modeled on Cargo’s crates.io [package] keywords grammar (the parser the crates.io publish API routes every keywords: entry through at publish time), narrowed to the strict ASCII subset every realistic search tag uses:

• 1..=CHART_KEYWORD_MAX_LEN (20) bytes;
• first byte: ASCII letter (A-Z or a-z). Leading digit, -, _, whitespace, control, and non-ASCII are each surfaced with a self-locating reason naming the canonical authoring footgun (paste-from-numbered-list "1foo", kebab-leak "-foo", snake-leak "_foo", paste-from-aligned-doc whitespace, paste-from-Unicode-doc non-ASCII);
• remaining bytes: ASCII alphanumeric, _, or - (Cargo’s crates.io-accepted continuation set; tighter than is_cargo_feature_name’s _/-/+/. continuation set — + and . are not part of the keyword grammar). Whitespace, , / / / ; / . list-separator confusions, control bytes, and non-ASCII bytes are each surfaced with a self-locating reason naming the canonical authoring footgun (multi-tag blob in one entry, CSV-list-belongs-to-list-grammar miscomprehension, CR/LF paste-from-doc, NFC/NFD normalization drift).

Returns the parser-shaped reason on rejection (without wrapping in any error variant) so each per-axis caller — crate::Caixa::validate_etiquetas for the universal :etiquetas axis at validate time, every future per-keyword axis (a future caixa-registry keyword-index lookup, a future Artifact Hub-keyword scraper validator, a future per-Aplicacao aggregated keyword set) — wraps the same reason in its own typed *Invalid { <axis>, reason } variant.

Empty input is rejected here (defensively) and at each call site via the narrower crate::ManifestError::EtiquetaEmpty variant — the same empty-first cascade is_dns_1123_label, is_gateway_api_http_path, is_wit_world_ref, is_nats_subject, is_wasi_keyvalue_slot, is_git_ref_name, is_git_oid, is_git_repo_url, is_cargo_feature_name, is_spdx_expression_shape, is_chart_description_shape, and is_chart_maintainer_name_shape all carry at their call sites.

Errors

Returns the parser-shaped reason naming the specific violation (length / first-byte-class / continuation-byte-class / whitespace / control-char / non-ASCII / ,-list-separator-confusion / /-path-separator-confusion / ;-list-separator-confusion / .-namespace-confusion), without wrapping in any error variant — every caller maps the same String into its own typed *Invalid { <axis>, reason } enum variant.